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Talk:Depth of field

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Contents

[edit] Old junk previously not in a section

In photojournalism we went over "circle of confusion" again--and after hearing for the 3rd or 4th time, I think I finally understand it. It has something to do with the diffraction of light as it passes through a lens, causing blur b/c of overlapping on the negative. I think. Could someone who knows, not suspects, go through this again, maybe providing an image to help illustrate it? Thanks, Koyaanis Qatsi

The circle of confusion is due to refraction, not diffraction. Generally speaking, if you have a point source of light in front of an (ideal) lense, then refraction and the particular shape of the lense causes light rays from the source to meet in a single point behind the lense. If that point happens to lie on the film, the point will be in perfect focus on the photo. If the point lies before (or behind) the film, then the rays haven't completely met yet (or are already diverging again) when intercepted by the film. The precise circle on the photo results from the fact that the light rays, before hitting the lense, went through a diaphragm of fixed aperture; without the aperture, the bright region on the film would be much bigger, since a lot more light rays from the source would contribute. AxelBoldt, Saturday, May 25, 2002

Refraction, right. I had the right idea but wrote the wrong thing. What I don't understand, though, is why having a wider aperture results in shallower depth of field and a smaller aperture results in greater depth of field. I keep thinking I understand it, and deciding I don't. Which is why I thought a diagram and an explanation would be nice. Koyaanis Qatsi, Wednesday, May 29, 2002

Making the aperture smaller makes the circle of confusion smaller. Imagine the ideal case: a tiny tiny aperture, letting only a "single" light ray through. That light ray would be refracted at the lense, but would stay a light ray, and the circle of confusion would be a single point. Now a little larger aperture will let several rays through, these diverge a bit, the lense brings them together again, but the film intercepts them "too early", and you see a small circle. The larger the aperture, the more light directions get through, the lense tries to bring them together again, but they hit the film too early and you get a larger the circle of confusion.

Now, all the distances for which the circle of confusion is small enough will be more ore less in focus on the film. If the aperture is smaller, a larger range of distances will qualify. AxelBoldt, Wednesday, May 29, 2002


The explanations above about circles of confusion and how they are related to aperture could be more precise. To explain the principle more concretely you need to think about how light rays are bent in order to focus them to a point on film. None of the explanations above makes this point explicitly. When the aperture is wider, the light rays need to bend at a greater angle in order to meet the film plane at a point (it is important to note here that the distance from the aperture to the film plane cannot be changed for the purposes of this example). It is this greater angle of refraction, and this alone, that accounts for the reduced depth of field. I will try to use simple keyboard characters to illustrate the point; imagine that these characters represent the aperture, the light rays, and the film plane:

            o >|<

The "o" is the aperture, the "|" character is the film plane, the ">" character is the light rays being bent onto the film plane, and the "<" character is the light rays as they would continue on after meeting at a point. Consider the light ray that starts at the upper left and descends to the lower right; label this line AB (“A” at the upper left, “B” at the lower right). Now consider the light ray that starts at the lower left and rises to the upper right; label this line CD (“C” at the lower left, “D” at the upper right). Lastly, label the point in the center where the light rays meet as “F”; this point where the light rays meet is the only point where the light rays are in perfect focus. Ideally, point F lies directly on the film plane. However, many points of light do NOT line up directly on the film plane. Because of their varying distances from the camera, many points of light that form a real-life image are bent to planes that lie slightly in front of or behind the film plane. To illustrate this concept, imagine the lines AB and CD moving together as a group slightly to the right. Now the point of perfect focus is no longer on the film plane; now the point of perfect focus is behind the film plane. Also note this: now the "|" character representing the film plane no longer meets lines AB and CD character at one point. Instead it meets the lines at two points. Two points form a line. If you draw in this line you have now drawn in your circle of confusion. The point of light in the real-life image is now no longer reproduced as a point, it is reproduced as a circle. However, this circle might still appear as a point to the human eye, depending on variables such as the size of the reproduction and the distance from which the reproduced image is viewed. When the circle becomes so large that it no longer appears as a point to the human eye, then it begins to appear out-of-focus.

Now you must consider how changing the aperture changes the circle of confusion. When the aperture is made wider it looks more like this "0" than this "o"; if the film plane is kept at a fixed distance from the aperture, then the light rays need to bend at a greater angle in order to meet the film plane at a point. Draw a diagram and make the circle (aperture) greater and you will see what I mean. The angles here that concern us are angles AFC and DFB (you can ignore angles AFD and CFB). As the aperture becomes wider, both angles AFC and DFB become greater. To illustrate the effect this has on the circle of confusion, you should try drawing two extreme examples: draw the first example with a very small aperture, the second example with a much larger aperture (remember to draw the film plane at the same distance from the aperture in both examples). In the first example, angles AFC and DFB will be relatively small, in the second example angles AFC and DFB will be quite large. HERE IS THE CRUCIAL COMPARISON: if the film plane is 1.0 millimeters in front of or behind point F, then the circle of confusion will be larger in example 2. If the film plane is 2.0 millimeters in front of or behind point F, then the circle of confusion will be larger in example 2. If the film plane is 3.0 millimeters in front of or behind Point F, etc. etc. This is the core of the aperture/DoF relationship. WrathofAbsalom 01:28, 8 February 2006 (UTC)


Is it worth commenting on the depth of field of a pinhole camera? David Martland 07:28, 10 Dec 2003 (UTC)

Sure

Aren't the Df and Dn equations messed up? It seems if s>f then the equations as listed always give Df<Dn. Seems backwards.

[edit] About the equations

I was conspicuous too. Therfore I searched trough some other sources. And there I saw our notion is correct. I didn't ever collaborate in a wiki before but I took the liberty to change the two equations and add another notation which is more readable for me. Is the righthand-notation better for everybody? Or is the lefthand used for a particular purpose? If so can somebody make the choice for me? --!nok 15:37, 14 October 2005 (UTC)

The equations were slightly wrong before, but even more incorrect after !nok's change. I have replaced them with the correct equations. Note that there are two different mathematical definitions of hyperfocal distance (they differ by a +f term at the end), and it's important to use the correct depth-of-field equations for the particular hyperfocal distance formula being used. I've also refactored the equations slightly, bringing the "S x" term down to make it a bit more obvious how the equations are structured... I hope. Doug Pardee 19:57, 6 March 2006 (UTC)

[edit] About the photograph

I was browsing through my old photos when I came upon this. I was experimenting with my camera's macro function while reading "The Camera" and I came up with this. When I found it, I thought it would be perfect for this article. Number one, speaking technically, it more clearly demonstrates "Depth of field" than the original photograph, and it's also larger, clearer, and sharper. And secondly, it adds a little humor to the article as well, because the words "depth of field" are within the sharp area of the DOF itself! The first sentance in the article states "In film and photography, the depth of field (DOF) is the distance in front of and behind the subject which appears to be in focus." and I believe my picture shows this vividly. PiccoloNamek 05:30, August 31, 2005 (UTC)

That's a great picture -- thanks for putting it in the article! The only thing that would make it better would be if the text read "A long time ago, in a galaxy far away..." :-)
Atlant 11:52, 31 August 2005 (UTC)
Agreed - love the picture - makes this article super-stylish. --DreamsReign 04:36, 17 May 2006 (UTC)

[edit] About the photos provided...

The article provides several photos for examples. The series of pictures under the flower are not a result of f-number change, but rather, of perspective distortion (the hitchcock zoom) I believe. The depth of field in all of those pictures are the same; the perspective distortion only makes the depth of field SEEM shallower as you progress down the pictures; but of course; this doesn't really matter, because the article describes these pictures as a result of f-number change or aperture change, which they are not. I may be wrong; so please correct me if I am.

Wouldn't the Cowslip photograph be more appropriate in the Artistic considerations section? JeffConrad 22:46, 15 October 2006 (UTC)

Agreed (it's my pic) and now moved - Adrian Pingstone 08:52, 16 October 2006 (UTC)

[edit] On the other hand...

Actually, I believe the photos are not misplaced; now that I look at them closely, I think they are actually shallower and in fact a result of the aperture changing. This picture acts as a double-optical illusion; normally, it's easy to tell between perspective distortion and depth-of-field change... Sorry; I got the idea that somehow the background seemed to be getting bigger and bigger; as if it was a result of the hitchock zooming effect; when in fact, it was just getting more shallow (and because they got so blurry, sticks started "disappearing" in the background, as if they were distorting). So everything is fine -- the pictures do in fact get shallower; and they are a result of f-number change; instead of what I had previously posted above (a result of perspective distortion and optical illusions). Ironic that I had thought it was an optical illusion at first... and was in turn fooled by another optical illusion.

[edit] An error and a suggestion

Error: text is missing below the first figure.

I don't see what you think is missing. What do you see it saying, and what would you suggest? Dicklyon 22:35, 2 June 2006 (UTC)

A suggestion: the discussion doesn't say that "N" in the equations stands for the f-number. Alison Chaiken 20:16, 2 June 2006 (UTC)

In the section "Depth of field formula" it says "Let H be the hyperfocal distance (calculated below from N, the f-number, and c, the circle of confusion for a given film format), ...". How would you recommend making it more clear? Dicklyon 22:35, 2 June 2006 (UTC)

[edit] New equations 28 August 2006

I've revised the DOF equations to make them consistent in form, and have attempted to show how some of the approximations are obtained, and that all equations derive from the same basic assumptions, with simplifications under certain conditions. I eliminated expressions using hyperfocal distance from the close-up formulae under the assumption that hyperfocal distance isn't terribly meaningful for close-up work (they're simple enough to restore if someone thinks they are of value). The first two such formulae appeared to be incorrect: if

H = \frac {f^2} {Nc} + f,

the H − f terms should appear in the numerator as well as the denominator. In any event, the other formulae presented seemed more than sufficient and more convenient to apply.

I also eliminated the center dots in the formulae: their inclusion seemed to be at odds with the Manual of Style, and certainly with conventional practice. I submitted a revision that kept the dots before making this change, so that they are easy enough to restore if someone feels they are absolutely necessary, though I think they served more to clutter than to clarify.

JeffConrad 02:40, 28 August 2006 (UTC)

Jeff, in a previous round of changes, I sought to use equations that are found in the literature and accurate enough, while being much simpler, and at the same time acknowledging the existence of more detailed equations whose accuracy is however limited by the factors they neglect such as pupil magnification.
I haven't really studied the new section carefully yet, but my impression is that by starting with the rather complex equations it will lose a lot of readers, before they get to the simpler ones. Think about it from that point of view and see if you think a different order of presentation could be made workable. I appreciate your effort on helping to clean this up, as you did with EV and LV and APEX system. Dicklyon 03:01, 28 August 2006 (UTC)
Dick, I had the same concern, and almost made a comment to that effect. Perhaps some simplification of the initial presentation is indicated, with more detail later to demonstrate that the basic equations weren't simply pulled out of the air. With this approach, I'd be inclined to keep the basic presentation even simpler than it was. The question is, "To what use would these equations be put, and which equations would be the most useful for that purpose?" A few equation candidates:
  1. Total DoF?
  2. Front and rear DoF?
  3. Near and far limits of DoF?
  4. Near:far DoF ratio?
  5. Focus and f-number from near and far limits of DoF?
The last equations are almost the only ones that I've ever used, yet they possibly are the least common in the literature. It long has been my impression that DoF discussions concentrating on the object side of the lens are primarily academic exercises; in practice, the task of controlling DoF usually is much easier on the image side:
  1. View camera users who calculate DoF usually use the focus spread (difference between near and far image distances) to determine focus and f-number.
  2. Small- and medium-format users typically use lens DoF scales to accomplish the same task. At least they did with manual-focus lenses ... except for a few older Canon 35 mm cameras (with the Depth-of-Field AE mode), there is no easy way to control DoF with most autofocus lenses. It's possible, at least in theory, to use object-side relationships in conjunction with lens DoF scales, but the resolution on most AF-lens distance scales is so poor that it's difficult to set the distance with much precision (e.g., it's easy to calculate hyperfocal distance, but tough to set it).
That said, I think getting into the image side probably would lose almost all but the really hardcore readers.
My personal observation on practical control of DoF would be something like:
  1. Determining DoF from focus spread or lens distance and DoF scales is reasonably straighforward for distances large in comparison with focal length; as noted, however, even this is no simple task with most AF lenses.
  2. Determining close-up DoF with unit-focusing lenses is feasible in theory but quite a chore in practice. With most current small-format internal-focusing long-focus macro lenses that change pupil magnification, internodal distance, and focal length with subject distance, it's almost impossible.
In other words, great accuracy is not needed to determine DoF at moderate subject distances, if your camera will allow you to do it. Don't try to calculate close-up DoF at home ...
JeffConrad 06:18, 28 August 2006 (UTC)

[edit] Larger formats -> smaller depth of field

I know that a larger aperture leads to smaller depth of field, but I don't quite see why longer focal length does the same - as written in the section Definition of "focus". Is it because the aperture has to be enlarged accordingly? Could someone spell this out to me? Perhaps I am just confusing the terms depth of field and depth of focus? Thursday, September 28, 2006.

Put simply, for the same subject distance, a longer focal length provides greater magnification, and to a first approximation, DOF is inversely proportional to magnification. Hence the reduced DOF. JeffConrad 08:03, 28 September 2006 (UTC)
Okay. Thanks for your quick reply. When I look at the f-numer equation
N = \frac{f}{d} ,
i still get confused, however. I read several places that increasing the f-number N increases the DoF. From the equation it looks like increasing the focal length f would have the same effect on the f-number as decreasing the aperture diameter d, which would both lead to an increased DoF. From your reply it seems that this isn't true. Is there simple explanation why?
I should mention that I have no background in optics or photography. My interest is only out of curiosity. Bade, September 28, 2006.
The answer is simple: N is not the only variable in the DoF equation. Dicklyon 13:41, 28 September 2006 (UTC)
I can see that, but if I kept d constant, would a larger focal length f (and thus larger N) result in an increased DoF? In other words: does the DoF depend ONLY on the ratio N, or does it depend also on the absolute values of d and f. Does (for example) N = 50mm/1mm and N = 100mm/2mm (same ratio) give the same result (DoF-wise and otherwise)?
Bade, Thursday, September 28, 2006.
Plug some examples into the equation of your choice and see what happens. In general, no; increasing f will give you LESS DoF, not more, if you keep same aperture diameter d. As Jeff points out, the magnification view makes this easiest to see, but any form of the equations should give similar results. Dicklyon 15:14, 28 September 2006 (UTC)
The magnification (and hence the focal length) and circle of confusion scale with the format. See the reference Jeff Conrad's Depth of Field in Depth (PDF) under "Depth of Field and Camera Format" for a discussion of how this affects DOF. JeffConrad 23:33, 29 September 2006 (UTC)

[edit] Depth of field versus format size

Dick, I got rid of the sentence

An 8x10 camera can be used to acheive the greatest depth of field and focus control, but at f-numbers such as f/64 the exposures can be extremely long.

because it didn't seem to make sense in the context in which it was used. Was the intent something to the effect of, "large-format cameras often can employ movements to achieve even greater DOF than smaller cameras"? If so, that probably should be mentioned. Of course, it also might be mentioned that a small camera can employ a tilt/shift lens to regain the advantage.

JeffConrad 07:58, 28 September 2006 (UTC)

I don't know the intent, as I didn't put that (I did edit it a bit). It does seem a bit narrowly put. I doubt that the movements were part of the intent, but that's also a good thing to mention. Dicklyon 13:41, 28 September 2006 (UTC)
So I see ... looks like it was Mr. Anonymous. Had I paid more attention to the history, I'd just have nuked the sentence without comment. Maybe it's best just to leave it out; it's contradictory to mention the greater DOF with smaller cameras and yet claim that the greatest DOF can be had with the largest image format. As the View Camera article mentions, using tilt or swing doesn't really increase DOF, but rather changes the plane of focus to better fit the DOF to the scene. We could add View Camera, Scheimpflug Principle, or Large Format to the "See also" section, but I'm not sure the treatment of movements in any of these articles goes far enough to explain how tilt or swing helps make up for the lesser DOF in larger formats. JeffConrad 22:44, 28 September 2006 (UTC)
What the heck ... I've added a two-sentence mention of movements and tilt/shift lenses to the end of the "Depth of field versus format size" section. I'm not sure that's where it belongs, but I can't think of where else to put it. An article on tilt/shift lenses remains a task for another time. JeffConrad 23:03, 28 September 2006 (UTC)

Isn't

"Consider formats that differ ..."

and the rest of the fourth paragraph a repeat of the previous paragraph? JeffConrad 22:37, 29 September 2006 (UTC)


In mentioning NIST Special Publication 811 in my last edit summary, I overlooked another obvious and more accessible source: the Wikipedia article on ISO 31-0. JeffConrad 22:59, 29 September 2006 (UTC)

Jeff, FYI, a Wikipedia article is never a source; they're OK for "see also", but not as references for sources. It's too transient, and needs to have sources of its own to be verifiable, so list a real source instead. Dicklyon 04:27, 11 October 2006 (UTC)
I chose my words in haste; I never intended to suggest the Wiki article as a "reliable source" in the formal sense, but rather as a guide to Wiki authors. NIST SP 811 obviously is an authoritative source in the USA; NIST SP 330 also is useful. The ISO 31 standards are the definitive references, but unlike the NIST documents, they aren't free. JeffConrad 04:54, 11 October 2006 (UTC)

[edit] Delete Circle of confusion computation?

I've listed Circle of confusion computation for deletion. Please take a look if you care, and leave a comment. Dicklyon 04:24, 11 October 2006 (UTC)

[edit] Close-up DOF

There are two headings with the same name both starting:

When the subject distance s approaches the focal length

This is a great article but the redundency needs to be cropped out. I just added an image demonstrating close-up DOF. HighInBC (Need help? Ask me) 18:31, 16 November 2006 (UTC)

Take a look at [this diff] and see if you can think of a better organization. The idea was to have a first simpler presentation, and a later more gory derivation. Dicklyon 18:46, 16 November 2006 (UTC)

Hmmm The divsion does seem to make sense. Perhaps the names of the headings can be disambiguated. HighInBC (Need help? Ask me) 18:48, 16 November 2006 (UTC)

I had the same reservations when writing this, but redundancy is an unfortunate consequence of having both basic and detailed presentations. There are two other subheadings that appear in both the basic and detailed sections, but given the hierarchy of the subdivisions, I don't really see the ambiguity. The primary headings (or some qualifying adjectives) could be prepended or appended to the subheads to give unique names, but I find this cure worse than the disease. I think elegant variation would be similarly inelegant. JeffConrad 23:17, 16 November 2006 (UTC)

[edit] Photo clutter

Is it just me, or does everyone want to add their own shallow-DOF photo to the article? The number of photos exhibiting the property is, for all practical purposes, infinite. Perhaps it would be better if we narrowed it down to a few less? Girolamo Savonarola 00:03, 17 November 2006 (UTC)

I did just add one, but I did so becuase I though an example of close-up DOF was needed. The related section has alot of text on the subject and no image. But mabye you are not refering to me hehe. HighInBC (Need help? Ask me) 00:06, 17 November 2006 (UTC)
Not you specifically; it's something that's been bugging me for a while. I guess the recent addition just sparked me finally commenting on it. The problem is that your image doesn't really show anything different from any other photograph with a shallow DOF - the first image in the article being a notable example.
The main problem, however, is that there are too many images period. I think that certain other ones may be worthy - such as an image progression showing the same image with different apertures. But beyond one or two examples, what else can you really show about DOF that is different? Let's decide what types of photos should be here and look for the best examples instead of adding photos which are good examples and trying to come up with a pretext for why it should belong to this article. Girolamo Savonarola 00:15, 17 November 2006 (UTC)
I completely agree with Girolamo—this isn't a photo gallery. The only justification for a photo in this article is its illustration of the concept. The best illustration of shallow DoF includes a photo of the same subject showing greater DoF (as do some of the first images submitted). I'm for weeding out the rest; they're gratuitous, detracting from the article rather than adding to it. JeffConrad 00:33, 17 November 2006 (UTC)
Well I am not to attached to the idea of the image being here. I don't mind if it is removed for housekeeping. HighInBC (Need help? Ask me) 01:27, 17 November 2006 (UTC)
Again, I agree with Giralomo that it isn't just your image. I'd also eliminate "A Cowslip flower ...," "Artistic effect ...," and possibly the kitten and the sequence below it. This may be going a bit too far, but again, the question is, "Does the image really illustrate the concept?" I think Paul van Walree's site is a good example of illustrative images that also happen to be well executed. JeffConrad 01:57, 17 November 2006 (UTC)
What many have failed to recognize is that the thumbnails have a whole lot more DOF than the full-size images, and thereby fail to make their point unless clicked on. We need examples that work in a small size. Dicklyon 06:43, 18 November 2006 (UTC)
Demonstrating rather conclusively that "apparent DoF" is redundant. It's tough to see what's sharp and what isn't in a small image (much like looking through the viewfinder of a small-format camera). The primary audience for images here probably are people relatively new to photography, so an illustration should be obvious: what is unsharp should be obviously unsharp, and what is sharp should be very sharp (preferably the result of a tripod-mounted camera). I think the images of the type, the butterflies, and the two white flowers make their points quite clearly; with the other images, the differences aren't as obvious. I think it's especially important for the differences in a sequence of images at different f-numbers to be obvious in the thumbnails, because it's awkward to move among the larger versions. JeffConrad 22:20, 18 November 2006 (UTC)

Although there hasn't been much response to Girolamo's original suggestion, no strong objections to reducing the number of images have been presented, either. It isn't possible to have this many images and still have them positioned near sections to which they relate. Unless someone has strong objections, I'm going to remove the Cowslip flower, the child, and the pen tip; I think the previous images adequately illustrate the concepts. I personally find the differences in the f/22 through f/2.8 sequence a bit subtle, but I'm inclined to leave them for now. JeffConrad 20:43, 15 December 2006 (UTC)

It would appear that, like the entropy of the universe, the number of images in this article cannot decrease. The process of culling the surfeit would seem unavoidably capricious; absent a strong consensus to the contrary, I'm going to leave things as they are. JeffConrad 22:14, 18 December 2006 (UTC)

I would say, if you haven't had any personal involvement with the creation of any of the images, delete whichever seem most appropriate. As for the sequence, perhaps it would be worth contacting one of the people in the image editing project so that they can be turned into a series of frames for an animation, thus saving the clutter of multiple images of the same thing. Girolamo Savonarola 22:19, 18 December 2006 (UTC)
Jeff, I'd go further and encourage you to "be bold" even if some of the photos are your own. You're the guy who has done the most to refine the content of the article, so there's no way your actions will be taken as anything but constructive. If someone objects to a removal, it can be negotiated, but I don't think anyone's likely to be too attached to their particular images here, or too touchy about trying to prune them. Dicklyon 00:17, 19 December 2006 (UTC)

I've removed the Cowslip flower, the child and the pen—I think some of the earlier images adequately illustrate the same concept. I would propose that we use the following criteria in considering whether to add images in the future:

  • The image should clearly illustrate some concept relevant to the article.
  • In this article, the differences between sharp and unsharp should be substantial, perhaps almost artificially so, because of the increased DoF of the thumbnails, but also to make the point readily apparent to someone new to photography. Ideally, this would not only make unsharp areas obviously unsharp, but also have sharp areas very sharp (i.e., tripod-mounted camera if possible).
  • Ideally, an image also should be pleasing and well executed.

I have some reservations about the image of the Wolf spider. Although it's well executed, it's not obvious what is being shown. Those who have done insect photography probably will recognize the greatly increased DoF, but this improvement may be lost on others. I think the image would be far more instructive if the photographer were to include one of the individual images to show how limited conventional macro DoF is.

Girolamo's suggestion about the animation may have some merit (who to contact?). I also think the differences among the images could be somewhat less subtle. I'll see if I can come up with a more graphic illustration, but I'm not sure when I'll get to it. JeffConrad 18:29, 19 December 2006 (UTC)

[edit] Edits of 11–12 December 2006

I agree with editor 208.104.120.140 that the second sentence under 'Aperture effects" was smoother without the parenthetical information. However, I also think that information is important, especially for newcomers or casual photographers, who often are confused by the inverse relationship between f-number and aperture size. The entire section probably would benefit from rewriting. JeffConrad 21:46, 11 December 2006 (UTC)

I've tried to put some of the more basic material closer to the beginning, and better group the images with the sections to which they correspond. Only so much is possible, however—as has been noted, there simply are too many images, several of which contribute nothing but clutter. JeffConrad 09:22, 12 December 2006 (UTC)

[edit] Introductory paragraph

I'm somewhat baffled by the last sentence in the opening paragraph, and am inclined to remove it:

This region is greater behind the point of focus than it is in front, because the angle of the light rays change more rapidly; they approach being parallel with increasing distance.

Is there something obvious that I'm missing? JeffConrad 18:29, 19 December 2006 (UTC)

It could be made more intelligible. I think this is what it means:

This region is greater behind the point of focus than it is in front, because the angle of the light rays change more rapidly with distance closer than the focus point than with distance further; rays approach being parallel with increasing distance.

Dicklyon 19:15, 19 December 2006 (UTC)
Your take is much the same as mine. The angle between the marginal rays from the more distant point always is smaller than the angle between the marginal rays from the closer point; however, this does not establish that the DoF behind the subject is greater than the DoF in front of it. It's simple to construct a diagram showing the far DoF less than the near DoF; such a diagram would, of course, violate the lens conjugate equation.
It's impossible to construct such a contradiction if you ignore the conjugate equation and just draw the "outside-the-box" rays by Moritz von Rohr's method. And it's not that the angle behind is less, but that it's less different from the angle at focus. That angle difference translates to a COC, pretty nearly. But, it's a complicated explanation that hard to see easily. Dicklyon 22:42, 19 December 2006 (UTC)
Perhaps the DoF distribution still is worth mentioning, though I wonder if it's necessary in the opening paragraph. In any event, I think the explanation needs to go. I know of no way to show the DoF distribution other than mathematically; it's easy enough to add this if it's thought that the benefit outweighs the clutter of additional math. JeffConrad 21:49, 19 December 2006 (UTC)
I agree it doesn't belong in the lead, since it takes too much space to explain that the distribution goes all the way from symmetric in macro mode to infinitely more behind is distant mode, with everything in between. In my paper, I explained four regions, one of which is the region around which the popular "rule of thumb" of about twice as much behind as in front is actually nearly true. Should we add explanation of those four regions? Or we could use a picture to illustrate somewhat more behind than in front. I'll see what I have... Dicklyon 22:42, 19 December 2006 (UTC)
My initial reaction, which I didn't state very clearly, was that the conclusion was far from obvious to me without a diagram or some other explanation. A diagram might address that, but at that point we might as well add a derivation. At the very least, we need something to establish the near focused, and far distances from the lens. Although this could be done on either the image or object sides, the concept of DoF necessarily starts with an image-side blur spot, so I think an image-side derivation would be simpler and more appropriate for this article. It's certainly not difficult, though the article already is getting a bit long. It may be simpler to add a few equations; moreover, we've already pointed reader's to several derivations, including two online versions.
I had thought of covering the near:far DoF ratio, but again thought the article was a bit long. Perhaps a simple explanation would suffice; I tend to view the ratio as a continuum, ranging from zero at the hyperfocal distance to a limiting value of unity at high magnification. In particular, the distinction between you regions 2 and 3 seems a bit arbitrary. The continuum works only when using the "exact" equations for near and far limits of DoF; the approximate equations
\frac {\mathrm{near}} {\mathrm{far}} \approx \frac { H - s} {H + s} = \frac { 1 - s/H} {1 + s/H}
would suffice for medium-to-far subject distances, but we'd need another equation, such as
\frac {\mathrm{near}} {\mathrm{far}} = \frac {fm - Nc} {fm + Nc}
(my Eq. 21, equivalent to your equations that follow Figure 3) for the macro region. I'll put something together in my user space to see if it's worth considering. JeffConrad 23:14, 20 December 2006 (UTC)
I have an expanded Basis of the DOF formulae section (which I've retitled) at User:JeffConrad/DoF equations. I think it addresses the issues raised, though I'm not convinced that we need all (or any) of it. Although it's convenient to have a self-contained derivation, the article keeps getting longer ... I am now convinced it is easier to show the near:far DOF distribution algebraically rather than with a diagram. Although it's easy to make a diagram showing greater DOF beyond the subject, it's not so easy to show that it must be so. JeffConrad 02:59, 21 December 2006 (UTC)
I'm against making an encyclopedia article too "mathy". You and I can follow all that and appreciate what it means, but to many readers it just becomes increasingly intimidating--a bigger chunk they have to skip and feel bad about. However, it might be good to summarize the result, that the 1/3-2/3 rule applies at H/3, and that the DOF is more symmetric when closer, more skewed when further. And I agree that it's easier to show the near:far ratio algebraically if you want to get quantitative, but I think it's easier to show diagramatically if you just want to show that it's greater on the far side. It takes a few words to motivate von Rohr's method, but then it becomes obvious, by construction, for whatever size circle you want to put in the field plane. Dicklyon 04:24, 21 December 2006 (UTC)
Depends on the article, I suppose, but as I said, I'm not convinced we need any of it. Quite frankly, I think the entire section Basis of the DOF formulae could be eliminated, pointing the mathematically inclined reader to any of the external links that cover this stuff in detail (I'd probably retain the image-side formulae for focus and f-number simply because they are among the few that actually are useful in the field). I see little reason to say anything about asymmetrical lenses except perhaps to mention that they aren't accurately described by the simple formulae. To my mind, the basic derivation would be far more useful (and less intimidating). In earlier edits, I simply retained many formulae that I probably would not have included in an article such as this.
One either gets into the mathematical detail or one does not, and when one does, the discussion gets quite lengthy, as both my paper and yours illustrate. The middle ground is tough to cover; I originally had a much shorter version, without formulae that I thought no one would ever miss. Of course, almost all the comments I got were about the formulae I had "overlooked."
It's easy to make a simple diagram using von Rohr's method, but really understanding it requires understanding the concept of projecting the image-side blur spot onto an object-side blur spot; easy enough, perhaps for Abbe, Kingslake, and von Rohr, but, to me, more difficult to grasp than the math, and definitely more obscure than the conventional image-side approach. The diagram on my page requires only first-year algebra and geometry, and the derivation certainly is no more complex than yours in the Circle of confusion article.
My original suggestion was simply to eliminate the last sentence of the introductory paragraph. Perhaps this could be tempered by adding a sentence elsewhere stating (but not demonstrating) something to the effect of
"The DOF beyond the subject is always greater than the DOF in front of the subject. When the subject is at the hyperfocal distance or beyond, the far DOF is infinite; as the subject distance decreases, near:far DOF ratio increases, approaching unity at high magnification. The oft-cited 'rule' that 1/3 of the DOF is in front of the subject and 2/3 is beyond is true only when the subject distance is 1/3 the hyperfocal distance."
Such a passage might be subject to challenge, of course, but that is always a possibility with a statement that is not supported. Again, however, the reader could be directed to one of the references or external links for substantiation. JeffConrad 07:50, 21 December 2006 (UTC)
OK, I'll let you worry about whether some simplification can be had by "overlooking" some formulae. I'll put von Rohr's method into his article, since it is, as you say, a rather unusual treatment for a mainstream DOF article. Dicklyon 16:16, 21 December 2006 (UTC)
I think putting von Rohr's method into his article makes perfect sense; perhaps the DOF article can include a link. Including the translation is a big help for those of us who retained little of our high school German.
I've revised the introductory paragraph, added a section Near:far distribution of depth of field, added the image-side equations to the DOF formulae section. I've left the derivation section for now, including the added material. See my further comments under that section on this page. JeffConrad 21:36, 21 December 2006 (UTC)

[edit] Showing DOF in front and behind

Here's a drawing I made to show how near and far limits can be found, and why the distance to the far limit is more than the distance to the near limit, from the focused field plane:

This is essentially von Rohr's method, but with my angular COC parameter e; I hope it's more clear than his drawings (see Moritz von Rohr). The entrance pupil has diameter d and is at distance S from the focused field plane that is presumed to image exactly onto the focal plane. Dicklyon 22:55, 19 December 2006 (UTC)

Dick, no question about the conclusion, which follows using either the image-side or object-side approach. However, I think the conclusion is far from obvious without considerable additional explanation. Might it suffice simply to say that the DoF is greater beyond the subject and approaches a 50/50 split at close focus?
That might suffice. It shouldn't take many words to explain the picture. It's a lot easier conceptually than the image-side approach that requires invoking the lens equation. Dicklyon 05:36, 20 December 2006 (UTC)
Incidentally, I think adding the Dominic Groß translation would make the Moritz von Rohr diagram much easier to follow. JeffConrad 01:21, 20 December 2006 (UTC)
OK, I'll add that. Dicklyon 05:36, 20 December 2006 (UTC)

[edit] Edit of 21 December 2006

I've added a brief discussion of the near:far distribution of DOF. I've also included the simplified image-side equations in the subsection Focus and f-number from DOF limits under DOF formulae.

[edit] Derivation of the DOF formulae

I've retained the detailed treatment of the DOF formulae for now; if nothing else, it will be in the history if we decide to delete it and someone later wants to restore part of it. I've added the derivation of the equations for DOF limits, and a subsection on the near:far DOF ratio. I think the detailed coverage is helpful to the reader who wants the additional information; the section is essentially an appendix, so the reader who has no interest in the derivation can easily skip it (if the section is eliminated, the choice is eliminated for everyone). However, others may have different opinions. If the detailed treatment is too much, I think the entire derivation section can be eliminated without seriously hurting the article; only a couple of notes would need revision. JeffConrad 21:57, 21 December 2006 (UTC)

[edit] Fancy Italic “f”

Dick, in this context, f is a quantity symbol, and as such, should be set in italics (see ISO 31-0 or NIST Special Publication 811). ANSI and ISO standards all follow this practice. JeffConrad 07:45, 25 January 2007 (UTC)

I don't find where it says that f in f-number is a quantity symbol. I thought it was just a name. Can you point me more specifically? And what about the long hooked f as used in f/#? Is that part of the standard? Dicklyon 02:57, 26 January 2007 (UTC)
I don't think there is any explicit statement, but it seems obvious that f is the symbol for focal length. ASA PH2.12-1961, Sect. 3.4.2, states
The symbol for relative apertures shall be f/ followed by the f-number.
The same statement appears in ANSI PH3.49-1971, again in Sect. 3.4.2. At least to me, it seems obvious that f/# indicates 'focal length divided by number', with f the symbol for focal length. I don't think there's really a long hooked f—it's simply an italic f. That this is so is illustrated in ISO 2720-1974, Sect. 3.1.2. ISO standards are set in sans-serif type; hence, f is simply in sans-serif italic (Actually, it's just oblique, because an italic front is both oblique and cursive. But this is getting a bit pedantic ...). JeffConrad 04:29, 26 January 2007 (UTC)
Jeff, my reading of the history is different. Long before the 1974 standard, the typographical standard of the long hooked f, not the oblique f, had been adopted for f-numbers as in f/8 where the f is literally focal length, but not at all standardized for the expression "f-number" where f is part of a name. I studied an awful lot of old books on this. I'm surprised to hear that the ANSI standard has an ordinary oblique f for both. Very strange. Dicklyon 04:58, 26 January 2007 (UTC)
Well, looking at books again, I must say the typography is much more varied and less regular than the simplified memory I just described. Still, I see no evidence that the f in f-number has ever been taken to be a quantity as it is in f/8. Some use a specifically different symbol, like F-number and f/8 (The Eye and Visual Optical Instruments By David A. Atchinson, George Smith). Dicklyon 06:02, 26 January 2007 (UTC)
Dick, your knowledge of photographic history far exceeds mine. No disagreement that actual practice is all over the map. Logically, though, I can't see how f-number derived from anything other than the quantity symbol for focal length, so I'd have to say that the folks on the ANSI and ISO committees got it right. They're also arguably fairly authoritative. Sidney Ray and Warren J. Smith also follow this practice, and they're fairly authoritative sources. I still wonder whether the long hooked f was anything other than just italics (which often produce a long hooked f, at least in serif typefaces). JeffConrad 06:31, 26 January 2007 (UTC)
Like this: f − number. JeffConrad 09:40, 26 January 2007 (UTC)

[edit] Principal planes

And principle planes are not the same as principal planes ... JeffConrad 22:13, 14 March 2007 (UTC)

[edit] Aperture diagram (edit of 19 March 2007)

I agree with Girolamo—having the aperture diagram in the aperture article is more than adequate. JeffConrad 19:31, 19 March 2007 (UTC)

[edit] Where is distance measured from?

The text below the Nikon lens' photo says this:

A 35mm lens set to f/11. The depth-of-field scale (top) indicates that a subject which is anywhere between 1 and 2 meters in front of the camera will be rendered acceptably sharp. If the aperture were set to f/22 instead, everything from 0.7 meters to infinity would appear to be in focus.

Now my question is: Is the distance measured from the front of the camera? I thought it was to be measured from the point in focus! Please correct me if i am wrong. 165.125.144.16 19:51, 19 March 2007 (UTC)

The measurements on the lens barrel (focusing/DOF scale) are normally taken from the camera's film plane. My old Canon SLRs have a symbol like this: -o- on the top plate of the camera indicating the position of this plane. (Quantities in optical formulas may refer to different origins, like the center of the lens.) -- Coneslayer 20:08, 19 March 2007 (UTC)
The formulae in the DoF article give object distance from the object nodal plane; at moderate-to-large subject distances, the distance between this plane and the image focal plane (i.e., the film plane) usually is negligible, but for close-up photography, the difference can be significant. JeffConrad 20:17, 19 March 2007 (UTC)
The important thing to note in practical applications, such as working as focus puller, is that lenses designed for film cameras give their focus markings from the film plane, while lenses designed for video markets, such as ENG, often tend to have focus marks measured from the front element of the lens. At the end of the day, though, this has no effect on the depth of field itself - only where the focus should be measured from. Girolamo Savonarola 18:10, 20 March 2007 (UTC)

[edit] Imperceptible vs. acceptable blurring

Dick, is "acceptable" the right term in the introduction? In this context, I think it serves more to confuse than to enlighten. It would seem to me that in the general sense, "acceptable" could apply to any blurring that met the aesthetic requirements of a particular image, even one employing substantial selective focus. Isn't the concept of DoF that the CoC is a blur spot indistinguishable from a point? I recognize that "acceptable" appears several other places in the article, but in most instances, it means, essentially, "imperceptible". JeffConrad 00:32, 10 April 2007 (UTC)

Perhaps it is confusing in that sense, yes. But while the concept is usually "imperceptible", the actual values used are typically somewhat larger than a just-perceptible blur, perhaps by a factor of two, aren't they? In any case, different people choose differenet CoC values based on what is acceptable to them, like how close to have to look to notice blurring on a big print. Tieing this to perception is a conventional fiction, I think. Dicklyon 01:10, 10 April 2007 (UTC)
I certainly don't see any factor-of-two cushion. A Snellen chart is based on 30 cycles/degree, which is 6.875 cycles/mm at 250 mm. A Snellen chart is very high contrast, so the usual rule of thumb (see Ray 2002, cited in the Circle of confusion article) is to reduce this to 5 cycles/mm (equivalent to a CoC of 0.2 mm) to allow for the reduced contrast of normal subjects. The tools available to Snellen admittedly were limited in comparison to those available today; people at the Smith-Kettlewell Eye Research Institute in San Francisco tell me that resolution on the order of 35–37 cycles/degree to a high-contrast sinusoidal target is more typical. Even so, I still don't see the big cushion when allowance is made for normal-contrast subjects. I've seen some very optimistic claims on the net, but I've yet to see one with a credible explanation or citation of a reliable source.
I don't buy the theory that conventional CoCs are nothing but remnants of 1930s film resolution. The idea behind DoF is the perception of sharpness under normal viewing conditions, and not necessarily the maximum that film or an electronic sensor can capture. If the latter were true, a 4×5 would use the same CoC as 35 mm, and making an outdoor 4×5 image would be all but impossible. I don't disagree with Merklinger's math, but his illustrative examples employ magnifications seldom encountered by anyone but the CIA or David Hemmings. Even absent subject motion, the idea that a CoC can be reduced to any arbitrary value is far greater urban legend than imperceptible blur at DoF limits. Eventually, diffraction overtakes defocus blur, and a few simple calculations suggest that this happens at values not much less than conventional CoCs.
The basic concept of DoF always has been a zone in which everything is perceived as sharp. I agree that the choice of CoC is somewhat arbitrary, but if conventional practice is to be questioned, I don't think the introductory paragraph is the place to begin the attack. For the casual reader, the extra qualification is likely to confuse. JeffConrad 04:06, 10 April 2007 (UTC)
I'm OK taking it out of the lead, but I think it should be admitted that the conventional COC for "sharp" has crept over the years from about 1/700 to 1/1000 to 1/1500 to 1/2000 of the format diagonal. The latter for 10+ MP pixel peepers, of course. Dicklyon 04:29, 10 April 2007 (UTC)
I changed it (differently; tell me if you like it). But then I noticed the first section head is Definition of "acceptably sharp". So shouldn't that notion be in the lead? Dicklyon 06:48, 10 April 2007 (UTC)
Technically, I agree with the "specified" viewing conditions, but practically, I think the use of "normal or specified" still is confusing in the introduction. I've recast things a bit in attempt to avoid the problem; I've also moved the heavy stuff from the intro to the first section to be less intimidating. The intro now is a bit sparse, but everything I tried was little more than fluff. I'm not happy with "misfocus" (in the first section) but I'm afraid that "defocus" would be a bit much for many readers. See what you think ...
This article has been somewhat hijacked by a couple of still photographers; I hope we're still OK with the filming people ...
The topic of "CoC creep" would seem better suited to the Circle of confusion article. Of course, it would be interesting to see why the criteria have changed; I've seen many values cited over the years without much explanation. I don't suggest that the criteria I put in the CoC article are infallible, but at least they can be related to basic principles. For full-frame 35 mm, a 0.03 mm has been around since the 1930s, and is about 1/1440 of the format diagonal. It might derive from the assumption of enlargement to the long dimension of a 8×10 frame. The 1/2000 criterion is new to me ... JeffConrad 09:24, 10 April 2007 (UTC)

I think most of us agree that strictly, we speak of “acceptable” sharpness according to specified criteria. As I've mentioned before, though, this seems a bit technical for the first sentence. Strictly, “area” is probably applicable when applied to a two-dimensional image, but it’s somewhat at odds with the physical concept of DoF as a range of distances from the camera. In any event, reducing the number of words probably improves the readability, and to me, “Apparent sharpness” is less unwieldy as a section title than was “Apparent sharp focus”. See what others think. JeffConrad (talk) 07:08, 25 April 2008 (UTC)

“Portion of a scene” or something similar works for filming and photography but seems less suitable for something like photolithography; of course, “in front of and behind the subject” arguably had the same problem. We could say “the region of object space that appears sharp in the image”, but that seems a bit much for the first sentence. JeffConrad (talk) 22:09, 25 April 2008 (UTC)

[edit] Camera Movements and DOF

I hate to add yet another section, but it seems better to introduce camera movements in one place rather than two. I've avoided the term "tilted plane focus" because it isn't standard (try a Google search to see what I mean), but I'm not sure there is a good term: "tilted focus plane" seems to be the winner of the obvious terms with 18 hits ... Good technical treatments of tilt and swing are few and far between, and the authors all seem to use different terminology: witness the term used to describe the axis about which the plane of focus rotates as the lens is focused: "counter axis" (Scheimpflug), "hinge line" (Merklinger) and "pivot point" (Wheeler). I usually call it a "pivot axis" or even "POF rotation axis" ... I've used "rotation of the POF" for the process because it seems most descriptive of what is happening; perhaps there is a more elegant way of stating it. JeffConrad 07:51, 24 April 2007 (UTC)

[edit] Diagrams requested

 It is requested that a diagram or diagrams be included in this article to improve its quality.
For more information, refer to discussion on this page and/or the listing at Wikipedia:Requested images.

This article would greatly benefit from diagrams illustrating the various formulas. -- Beland 18:56, 5 May 2007 (UTC)

I've indicated DOF in the diagram in the section 'Derivation of the DOF Formulae', and directed the reader to that section. That diagram illustrates most of the quantities discussed in the basic section, and the picture of the butterfly gives an alternative illustration of the concept. It's certainly possible to provide additional diagrams, but the article is already a bit cluttered. Perhaps others disagree, however. JeffConrad 00:49, 6 May 2007 (UTC)

[edit] Edits of 5–9 June 2007

Try as I might, I cannot decipher the paragraph that was added to the Artistic considerations section (originally under Word of caution). Although I can think of several things that may be intended, I am guessing at best. Consequently, I cannot see what this paragraph adds to the article in its current form. Absent a clarification, I am inclined to remove it. JeffConrad 21:26, 6 June 2007 (UTC)

I think he was trying to say that the degree of blur of distant background is not determined by the DOF. But I agree it's a mess and hard to see how to fix, which is why I only took out the section head and waited for you to find it. Dicklyon 00:21, 7 June 2007 (UTC)
Actually, I held off waiting for you to make some cuts :-). If I knew how to fix it, I would do so. My guess was that the intent was to say that the amount of the background or foreground blur doesn't quantify the DOF. Perhaps the intent was simply that foreground/background blur is somewhat of a separate issue from DOF. I'm not sure why we would need this comment, though. When people attempt an attractive background blur, they don't usually give much thought to quantifying DOF (Merklinger's example of blurring a sign is special case). Perhaps the editor (WalrusJR) can clarify, but absent this, I think we should eliminate the paragraph. JeffConrad 01:43, 7 June 2007 (UTC)
I've tried to interpret the edit of 5 June 2007 and make the wording more comprehensible. Although, strictly speaking, the added paragraph is correct, we're really splitting hairs, and I wonder if the distinction merits this much space. I avoided the use of "nicely", because it could be taken to include bokeh. If it is felt important to cover the appearance as well at the amount of background or foreground blur, I suppose yet another sentence could be added, but I would wonder if we're getting too far off topic. JeffConrad 23:34, 9 June 2007 (UTC)

[edit] DOF and camera movements—edits of 8 June 2007

There is no maybe about the near and far limits of DOF no longer being parallel when the POF is rotated, as the Scheimpflug principle article or any decent text on view cameras will show.

The meaning of the added paragraph isn't clear. Perhaps the intent is at using swing rather than tilt, but either swing or tilt involves rotation of the POF, so it's not clear what is meant by alternatively". "Width of field" is an invention, even if a clever one, and doesn't seem appropriate for a WP article (should we also say "height of field" when tilt is employed?). If people think this article should say more about movements, it's certainly possible to describe the application of both tilt and swing, but I think such material would be more appropriate in either the View camera or Scheimpflug articles. Even a simple treatment will require several paragraphs and a couple of diagrams, and this article is already a bit on the long side. But whatever people think is appropriate. JeffConrad 20:42, 8 June 2007 (UTC)

[edit] Peer review and FAC?

Would any of the primary editors be interested in shepherding this article through the peer review and/or FAC processes? Girolamo Savonarola 20:50, 8 June 2007 (UTC)

If you think it's ready, and we get a couple more votes ... though I think a couple of minor details (discussed in the last few days) should be resolved first. JeffConrad 23:47, 8 June 2007 (UTC)
It can go into PR at any time, regardless of the state it's in. I would do it myself, except that I don't feel as familiar with the optics details nearly as much I'm certain a few people including yourself do. Girolamo Savonarola 00:22, 9 June 2007 (UTC)

[edit] Foreground and background blur—edits of 10–16 June 2007

The ratio of background blur to background object size is very similar in concept to Merklinger's object field method, considered on the image side of the lens. The key difference, at least in the context of this article, is that, within the DOF, the blur is imperceptible, so although the ratio still holds, it is irrelevant. In the object field method, the ratio would still be important.

The difference in concept could be regarded largely as a matter of viewing conditions; most examples showing the benefits of the object field method are at much greater magnification than would normally be employed. Alternatively, the difference could be regarded as what the imaging medium is capable of recording rather than what is noticeable under standard "normal" conditions of enlargement and viewing.

A brief mention of some or all of this could be added to the article if people think it is important. However, unless lack of mention is likely to prompt the question, I'm inclined to omit it because the article is already quite long. JeffConrad 04:48, 11 June 2007 (UTC)

I should have mentioned that the comparison in the second paragraph with the object field method applies only to distant objects; although the object field method is scene specific, the criteria for near objects often are less restrictive than the criteria of the traditional approach. JeffConrad

I've restored the comment on the ratio of background blur to imaged detail size because this also is objectively true; I've left out the interpretive comment, however. I also removed the interpretive comment from the Background and foreground blur subsection under derivation.

  1. Without mention of the ratio, there is a concerted effort to claim that a longer focal length produces greater blur, which doesn't seem NPOV and may or may not be true. No disagreement that absolute blur is proportional to focal length, but to mention only that seems a selective inclusion of facts.
  2. Few backgrounds, including those in most of the images in the article, are at infinity, so concentrating on the effect at infinity is of questionable relevance to to most photographic situations.
  3. Omitting mentioning the blur to detail ratio while retaining the link to van Walree's article is potentially confusing, because it's exactly what he discusses.

Using "detail" rather than "object" also seems less precise (the behavior applies to any object at that distance), but I've left it for now. JeffConrad 23:08, 11 June 2007 (UTC)

Strictly speaking, the pupil magnification cancels out of the expression for blur spot diameter if the distance is measured from the lens entrance pupil rather than from the object nodal plane. However, we've expressed all other formulae in terms of the distances from the nodal planes, so it would seem reasonable to do so here as well; I used "lens" in attempt to avoid the issue altogether. If people insist on referring to the entrance pupil here, I suppose I won't quibble, but I think by doing so we would confuse more than we would enlighten. JeffConrad 01:43, 12 June 2007 (UTC)

Jeff, you restored part of what I took out, leaving "For a given subject magnification, detail distance, and f-number, the degree of detail blur is proportional to the focal length." It's very hard for me to imagine what could be changing here with focal length if you're keeping subject magnification and background detail distance fixed; you'd have to be moving the background relative to the subject as you change focal length and move the camera to keep subject distance fixed. So I can't really find any truth in it. That's why I reduced it to only the special case of an infinitely distant background. And the next bit "however, the magnification of the detail is also proportional to focal length, so for a given detail, the ratio of the blur disk diameter to imaged size of the detail is independent of focal length," that's also not true, since m is not proportional to f in general, and it's introducing a concept that might make sense to Merklinger, but not to anyone else. I think we should drop it. Dicklyon 04:18, 12 June 2007 (UTC)

Dick, the magnification of the defocused object changes with focal length:
m_\mathrm d = \frac f { u_\mathrm d - f }
For u_\mathrm d \gg f, the magnification is essentially proportional to focal length. The ratio of blur spot size to imaged object size is constant with distance, for all distances. The proof, which I put in the derivation section, is very simple.
The only constraint that I restored was that of constant subject distance, emphasizing that the relationship between focal length and blur diameter applies at all background distances rather than just at infinity. Most backgrounds aren't at infinity, so I think this is important.
You mean constant subject magnification, yes? That's what it says, anyway. And in this case, the conventional approximation is that the DOF is nearly independent of focal length, meaning the blur diameter is constant, not proportional to f, at those distances at the edge of the DOF. I realize that's not exact, but it's certainly nothing like proportional to f. One of us is confused. Dicklyon 07:08, 12 June 2007 (UTC)
The only reason that background blur spot size is of interest to the average photographer is for isolating the subject from a busy background, and I believe that is what led us to add the section on background blur. The subject is isolated from the background because the background is blurred and the subject isn't; what's not so easy to determine is how blurred the background must be for adequate isolation of the subject. It seems a general rule of thumb that when the blur spot is approximately the same as the imaged object size, the object is difficult to recognize. I'll admit here that I'm relying on John Williams's Image Clarity, and he's really a secondary source—I haven't directly consulted the references in his bibliography. In any event, the concept didn't originate with Merklinger; where he differs from most others is that he applies the concept within the DoF, where most others (including me) consider the blur undetectable, at least under normal conditions of enlargement and viewing. Merklinger's method well may be appropriate for surveillance photography, where the main objective is in identifying objects, employing the maximum usable enlargement.
Certainly, reducing recognizability of the background plays a part in determining isolation of the subject, though I don't think it's as clear cut as Merklinger's example of the "PROWLER" sign on his neighbor's RV, i.e., I don't think it's essential that the background be completely unrecognizable. However, I think it's just as speculative to imply that absolute blur size is the only criterion as it would be to insist that a background object be unrecognizable. Van Walree's examples illustrate this quite well; to me, the image with the long-focus lens appears better separated from the subject, but I'd maintain that it's because of the narrower angle of view than the amount of background blur. This is entirely consistent with my experience in macro photography with long-focus lenses—though I like them primarily for working distance, they're also mighty handy for hiding a busy background simply because they include less of it. I never gave much thought to greater background blur, because I've never been convinced there's much subjective difference compared with a lens of shorter focal length.
In summary, if we're going to have a section on foreground and background blur, I strongly think we should mention the ratio of blur to imaged object size. I don't suggest that it's always the governing factor, but I think it's often as valid as absolute blur size, and to mention one without the other would be very disingenuous and misleading. I originally included the subjective comment just to make clear that neither criterion was necessarily absolute. We could add a comment about differing angles of view; although it's a bit off the topic of DoF, it's probably no more so than many other comments in the article, and in any event, it's an important consideration in isolating a subject from the background. JeffConrad 06:55, 12 June 2007 (UTC)
OK, but let's clear up the objective part that separates us first, and reference Williams on the other.
What is the objective part on which we differ? That should be simple to fix. As for referencing Williams, I'm not sure that's appropriate, because the criteria he cites are quite specific, and relate to legal criteria for identification of suspects and admissibility of evidence. I don't suggest that it's anywhere near that clear cut, but simply that the blur disk size, without consideration of its relationship to the object itself, isn't the entire picture when it comes to isolating a subject. Isolation of subject from background is necessarily subjective; although blur disk size ostensibly is objective, its relationship to background isolation is the only justification for its inclusion in the article. Perhaps a one-sentence comment would adequately cover what I had said in three or four sentences. JeffConrad 07:44, 12 June 2007 (UTC)
I've made a few subtle but important changes to the text; in particular, I've eliminated reference to "proportional". I've also eliminated the "however" in introducing the ratio of blur to object size, so the presentation should be more neutral. Everything in the section is objective, and now, hopefully, strictly correct. Although it might benefit from a sentence or two mentioning why the two measures of background blur were included, I think the material could stand as it is. See what you think.
I've expanded the treatment in the subsection under Derivation to support the claims of how the blur disk changes with focal length. JeffConrad 12:42, 12 June 2007 (UTC)
Thanks. The "proportional" was the main problem, since it didn't account for the variation in s and D. Dicklyon 14:38, 12 June 2007 (UTC)
I am afraid that the present text ("For a given subject magnification, f-number, and distance between the detail and the subject, the degree of detail blur varies with the focal length (varying s and D together as focal length is changed).") is not entirely clear. Somehow the article should say in words that the influence of focal length on the blur disk diameter is small when D is close to s, but sizable when the two quantities are well separated. Odo Benus 17:49, 12 June 2007 (UTC)
Yes, I agree. It's what the equation implies, but it's not yet clear, and it really is the point. For D not too far from s, the D in the denominator nearly cancels the f and there's not much effect, but for D much greater than f, the D in the numerator starts to cancel the one in the denominator, leaving the f to make a difference. Maybe a plot or examples would help. Have a go at it. I'm still not convinced that Jeff's focus on the size of background elements is very relevant to the objective issue here. Dicklyon 18:01, 12 June 2007 (UTC)
The ratio of blur to object size is every bit as objective as the absolute blur; it's just a different criterion. Whether one criterion is better than the other at "isolating the background" is necessarily subjective. I don't think I've ever seen a definitive authoritative statement that quantifies what isolates a subject from a background; van Walree makes as good a case as I've seen. I'd arrived at the same conclusion mathematically, but his images make for a more convincing presentation. One advantage of the ratio is that it's independent of focal length and subject distance, depending only on the defocus of the foreground or background. I don't suggest that it's the "right" criterion, and don't really care whether or not I can read a license plate—I just want the background not to be distracting. There simply is no way to make that an objective call.
I find the statement "varying s and D together as focal length is changed" a bit confusing, because we really aren't actively varying D. Assuming we're dealing with given scene with fixed positions of subject, foreground, and background, D varies with s simply because the its distance from s is fixed. It may be easier to illustrate this by calling the distance between subject and foreground or background object the defocus, given by
x_\mathrm d = \left | D - s \right |
We then could speak of fixed xd, and
b = \frac {fm} N \frac { x_\mathrm d } { s \pm x_\mathrm d}
Using the ratio of blur to object size,
\frac b { m_\mathrm d y } = \frac m { m + 1 } \frac {x_\mathrm d } { Ny },
which clearly shows the dependence on defocus and object size. I don't suggest that this is the "right" criterion, but simply that it may be as valid as any other.
It's certainly possible to add a plot, but with all due respect, of all of the many quantities that could be plotted, this would seem far from the top priority. I'm not even sure what a plot would really illustrate. It seems to me the implied question is, for a given scene and subject that I want to photograph at a certain magnification, what lens will give the best background blur? If the absolute blur is the criterion, the longer focal length certainly will give the best blur. But why would I want to blur the background? Presumably, to isolate the subject, and toward that end, I'm not sure the objective criterion is that obvious. Images make the case far better than formulae or plots, as van Walree's treatment shows. The effect of defocus distance could be illustrated with examples somewhat more subtle than van Walree's. I would not be surprised if the long-focus lens gave better isolation, especially with increasing defocus. However, I think the result would be due as much to the narrower angle of view than anything else. There would be one obvious result—it's nearly impossible to isolate a subject from a nearby foreground or background. But isn't this almost self evident? There already is a statement to this effect in the section on Artistic considerations.
Photographing the same subject with different focal lengths will result in differences in blur spot size, differential magnification with distance, and angle of view; all contribute to isolation of subject from background. I think it's a bit arbitrary, and possibly misleading, to concentrate on one to the exclusion of the others.
But in an article on DOF, it would also make sense to ignore things other than how much blur you get due to misfocus. We can't know what size background objects there will be, or whether more or less magnification of them will be a good thing, which is why I didn't take discussing them to be "objective". I do like your new equation that suppresses D in favor of the separation. Dicklyon 23:07, 12 June 2007 (UTC)
Maybe yes, maybe no. By definition, DOF separates what is perceived as sharp from what is perceived as unsharp; it's unavoidably subjective. We make certain assumptions about visual acuity, enlargement, and viewing to estimate what "appears to be in focus"; if we were truly objective, we'd not be able to arrive at an "acceptable" CoC, and we'd be unable to define DOF at all. Ostensibly, blur diameter is completely objective, but again, we'd have no reason to even explore the topic were it not for the unavoidably subjective issue of isolating the subject from the background—if you will, the degree of separation of sharp from unsharp.
It's easy to crucify a subject on the cross of objectivity. Recall 30 years ago when audio equipment manufacturers focused on minimizing total harmonic distortion, an objective and easily measured quantity, and touted THD on the order of a thousandth of a percent, when it was easily demonstrated that hardly anyone could detect THD on the order of several percent. The obsession with minimizing THD led many manufacturers to virtually ignore other issues such as phase shift and limited slew rate, which were easily demonstrated to have very noticeable effects on perceived sound quality. Like THD, both phase shift and slew rate were objective, easily measured quantities—it was largely a matter of what someone considered important and decided to measure. The situation here may be analogous.
In any event, I've given the "relative blur" only one sentence, and have not said anything that isn't strictly correct. I haven't suggested that any particular amount of "relative blur" is good or bad; I've pointed the reader to van Walree's article, and the reader can decide for herself whether it's relevant. JeffConrad 03:10, 13 June 2007 (UTC)
I also wonder if we aren't getting carried away with this topic (I'm certainly as guilty as anyone) and giving it far more emphasis than it merits. This isn't to say that it's unimportant, but so are most of the other sections. The article is already fairly long, and several comments have been made about the surfeit of images. JeffConrad 22:54, 12 June 2007 (UTC)
Sure we are. That what wikiaddiction does to you. Dicklyon 23:07, 12 June 2007 (UTC)
Dick, the "proportionality" holds only for fairly distant background objects; with the expansion to include foreground objects, it totally breaks down, and I neglected to adapt for the change. One way to use "proportional" in the restricted sense and still cover backgrounds short of infinity might be to say that, for a reasonably distant background,
b \approx \frac {fm} N ;
we rely on many similar approximations to arrive at the simplified equations. JeffConrad 22:54, 12 June 2007 (UTC)
Yes, that's the thing to do. I figured it was obvious when I wrote the version that only mentioned the infinitely distant background, but you're right that being explicit about the approximation is a good idea. Dicklyon 23:07, 12 June 2007 (UTC)
I'll make that change. Upon further thought, Odo Benus's suggestion to make the comment in the text probably is the simplest approach—one sentence should suffice. I'll also add the defocus distance that I mentioned and see people think. JeffConrad 23:40, 12 June 2007 (UTC)
Well, I took two sentences, but in them I also stated some assumptions that may have been obvious to us but not to the casual reason. I hope the current form addresses Odo Benus's concerns; I don't show the basis for the claims, but I don't know how to do so without substituting for the subject distance, and I think that would be getting pretty complex for the "basic" formulae section. In any event, see what you think. JeffConrad 03:10, 13 June 2007 (UTC)
Jeff, thanks. I think it's much better now. Dicklyon 04:11, 13 June 2007 (UTC)
Yup, it is much better now. However, the sentence "Outside the DOF, the blur increases with the distance from the subject." may confuse people if they interpret "Outside the DOF" as a condition for the blur increase. For the blur also increases with the distance from the subject inside the DOF. Odo Benus 16:33, 14 June 2007 (UTC)
Hopefully, the last change will eliminate the potential confusion. The case could even be made for giving the ratio b / c, with the implication than when b / c < = 1, b / c = 1. One could describe the blur in terms of CoCs; such a ratio would format independent. I'm not sure that it would add much in this context, though.
On unattached participles: put "b" after "differentiating" if you must, but I don't think it would add much—there is no ambiguity about what is being differentiated. I had originally thought of using "differentiation", but, like many -tion words, it seemed unnecessarily abstract. Use of the participle in this context in mathematical texts is common. The objection to unattached participles usually derives from ambiguity; that certainly isn't the case here. JeffConrad 23:28, 14 June 2007 (UTC)
Ambiguity arises with wrongly attached participles. Your participle is unattached in a "sentence" without a noun. It is true that dangling participles commonly appear in mathematical texts; that is why the author instructions of many scientific journals expressly warn against them. Anyway, there are probably more important things to discuss. Odo Benus 11:31, 16 June 2007 (UTC)
Many expressions like this have long been accepted usage; it could even be argued that "differentiating" is a gerund in this context. In any event, let's not quibble about whether this particular usage is acceptable; I've added the "b", which is also common form for mathematical texts. I've also added a sentence explaining the significance of the sign of the derivative; if it's superfluous, get rid of it. JeffConrad 21:47, 16 June 2007 (UTC)

Not all limited-DOF photographs involve separating flowers or cats from distracting backgrounds. Upon further thought, it seems to me that some mention needs to be made of the recognizability of objects, as much for situations in which an object must be recognizable (e.g., evidence and surveillance photography) as for situations in which they should be unrecognizable—I've made many photographs of the former type myself. It would be nice to look at one of Siljander's books, but absent that, Williams probably would suffice as a reference. I'm not sure the discussion belongs with the formulae, however.

On a related note: would the section "Artistic considerations", which initiated the fascination with foreground and background blur, be better titled "Selective focus"? This isn't to suggest that artistry isn't involved, but that "selective focus" is the common term (at least in the U.S.A.), and its use makes content of the section more obvious to a reader scanning the article. Also, is "differential focus" sufficiently common in British English that it should be mentioned as a synonym? JeffConrad 00:17, 15 June 2007 (UTC)

Good idea on the new section title. I hadn't heard of "differential focus", but looks like it is pretty common. Thanks again for all your concientious work on this article. Dicklyon 03:52, 15 June 2007 (UTC)

[edit] Chabacano's new figure

This figure is interesting. Mighty large, and not particularly helpful at illustrating where the size of the blur circle comes from, since the crossing of rays in front of and behind the focal plane is a tiny hidden detail. For now, I'll just shrink it a bit and copyedit the caption. Dicklyon 17:29, 15 June 2007 (UTC)

I tend to agree; the diagram is certainly more attractive than mine, but I'm not sure that it really illustrates what's happening. When the request for additional diagrams was made, I had thought of adding a couple of diagrams similar to the one in the Derivations section, but less cluttered. This could be added if people think it it would help.
The captioning illustrates once again the rather casual alternation between DOF and background blur. It's hardly unique to here; a quick review of every photographic text that I have confirms that for good or ill, it is almost universal practice to speak of employing shallow DOF (limited DOF/selective focus/differential focus) to isolate a subject from the background. Consequently, I think that's the terminology we should use. Though DOF and blur are different quantities, shallow DOF and background blur are really two ways of looking at the same thing. What usually isn't stressed is that the amount of background blur also depends on the distance between the subject and background. I'm working on a way to say this succinctly; perhaps a diagram also would help, though getting a reasonable scale might be tough because of the horizontal space required to illustrate the distant background. I'm also mindful of the comments about the clutter of too many images; I assume that, at some point, this would apply to diagrams as well. JeffConrad 19:48, 15 June 2007 (UTC)

Whatever the figure's merits, I find it rather confusing in the subsection "Focus and f-number". Logically, it should be close to the section "Effect of f-number", but without some further shrinking, I can't find a graceful way of putting it there. JeffConrad 22:07, 16 June 2007 (UTC)

Hi, I drew this figure because I tried to understand the relation between aperture and DOF and after browsing the internet some time I did not find good simple figures and explanations for it, so finally I did this draw for es.wikipedia, and eventualy placed it here too. I thought that an isometric diagram would be better, because I have noticed that most of people just skip standard optical diagrams and formulae, as they find them obfuscate, or hard and then they just memorize recipes like "big aperture->small DOF" as if it was product of magic :) (that is the only explanation I can came up to explain why so many photographers do not understand what is going on with this things apart from "reducing aperture will increase depht of field" when the try to explain it in their blogs or home pages). If you have improvements, please tell me and I will modify the diagram. Maybe the crossing lines in front of and behind the image plane should be emphasized in the caption, or maybe a mark (a small black circle) could be placed to emphasize them. I also dislike it in the focus and f-number section, but I couldn't find any better place. Finally, if you think that would be better to remove it, do it without hesitation.
Also, if you have more ideas for diagrams I would be happy to draw them. Thanks for the copyedit. Chabacano 03:40, 17 June 2007 (UTC)
If you could work on clarifying where the rays intersect the focal plane, and make the fuzzy spots more disk-like, it might help clarify where the circle of confusion comes from. Thanks for offering. Dicklyon 03:56, 17 June 2007 (UTC)
I've made yet another copy edit in attempt to say what the figure really shows; if we're not there, I think we're certainly closing in. I'm assuming that we really don't care whether how aperture size is decreased. I agree with Dick's comment, but I think perhaps the greatest value of the figure is in showing that the spade, heart, and club all appear sharp with the smaller aperture. JeffConrad 07:45, 17 June 2007 (UTC)

I was thinking in adding this closeup of the details, although maybe it just adds complexity. I tried to make the disc thing by increasing the size of the blurred spots. It is less real (now the blurred spots are "bigger", not only blurred), but maybe is more comprehensive and no noticeable. What do you think, better or worse? Thanks for your comments. Chabacano 10:13, 17 June 2007 (UTC)

The closeups certainly are illustrative, but they do add complexity, as you noted. The depiction in the main figure now is much better, though—I think if you simply extended the red rays to meet, the illustration of blur would be more than adequate, especially when viewing the full-size figure. JeffConrad 19:25, 17 June 2007 (UTC)

[edit] Accuracy

The diagram discussed above appears to be slightly inaccurate. The circles of confusion are, in the SVG, drawn as blurred ellipses. I'm not sure what convolution kernel the SVG standard calls for for "blur", but it looks like it's probably Gaussian, not a circle. The circle of confusion should be drawn as a lighter circle not as an amorphous blob. —Ben FrantzDale (talk) 17:20, 26 January 2008 (UTC)

[edit] Circles?

I'm not well versed in optics and am reluctant to tinker with this excellent article. But something strikes me as an oversimplification. We read: Precise focus is possible at only one distance; at that distance, a point object will produce a point image. At any other distance, a point object is defocused, and will produce a circular image (my emphasis). I believe that this is only true if the limiting aperture (in photographic applications, normally the iris diaphragm) is circular. In Image:Diaphragm-detail.png the diaphragm consists of a number of arcs that approximate a circle fairly well; however, in plenty of cameras that I have encountered -- notably early examples of "shutter-priority" automatic exposure (the photographer sets the speed, the camera sets the aperture) -- this is not so, and out-of-focus point sources of light are rendered as blurry octagons or even pentagons.

The crudest diaphragm would be triangular, and somewhere on the web there's an article that shows what happens to out-of-focus light sources via a triangular diaphragm.

And with mirror lenses you'll have what approximate to annuli (rings), though of course these are indeed circular. -- Hoary 00:53, 12 September 2007 (UTC)

Good point. The assumption of circular aperture is the conventional simplifying assumption, and we should just say so. Dicklyon 00:57, 12 September 2007 (UTC)
<homer>Mmmmm... Donuts of Confusion...</homer> -- Coneslayer 17:52, 12 September 2007 (UTC)

[edit] Link to olympuszuiko.com

In a sense, I guess the link is from olympuszuiko.com, though it's actually to a personal blog by the registrant of that domain. I think we already have plenty of these links—we're starting to approach link spam. If someone disagrees and thinks the link should be restored, though, I won't quarrel over it. JeffConrad (talk) 23:30, 11 December 2007 (UTC)


[edit] Diagram I made

I made a diagram to replace the fence and flower examples. I don't want to just replace it since it seems like a large change to the page, and I don't know if this is better. The fences don't really help a lot, and here it is easy to see the difference, and I have f/32 as well. I have 2 versions, the rolled-out and the animation, I don't know which people like more. They are of the same images, but the gif had to be indexed. Only one of these should go on the article, of course.

DOF at various apertures, from f/2.8 to f/32 using a macro lens at the minimum focus distance.
DOF at various apertures, from f/2.8 to f/32 using a macro lens at the minimum focus distance.

Hustvedt (talk) 02:39, 19 December 2007 (UTC)

I can't make much sense of the pictures. Without a familiar subject, it's hard to get a good sense of the depth. Dicklyon (talk) 07:14, 19 December 2007 (UTC)
Hmm, that is quite true. It is more apparent that it is a series of the miniature Christmas tree lights when the aperture is at f/32, so would it be better if I reversed the order of the images, or should I look for something else that would be better? Hustvedt (talk) 17:34, 19 December 2007 (UTC)
I don't think reversing the order would make a great deal of difference. I think the flower images are fine; the biggest objection to the fence images is that the DoF differences aren't quite as obvious as we might like. I agree with Dick that a reasonably familiar subject is essential. JeffConrad (talk) 23:18, 19 December 2007 (UTC)
Ok, how about this? I went for as simple as possible, and here I have a ton of contrast to make it easy to see what is going on. I hope you guys like this version. I only made an animation because the tall version is just huge. Hustvedt (talk) 02:56, 20 December 2007 (UTC)
This diagram illustrates some interesting behavior, such as the light and dark inversion at small f-numbers, but this is pretty esoteric for the average reader. Again, a more familiar subject (preferably, non-macro) would be much easier to comprehend. The two flower images sufficiently illustrate what happens with macro images. JeffConrad (talk) 20:44, 20 December 2007 (UTC)

[edit] New section Obtaining maximum DOF

Although the purpose of using the hyperfocal distance or the object field method is probably obvious to most of the major contributors, it may not be so to the average reader. Hopefully, the new subsection clarifies the purpose. I eliminated the reference to bokeh because it really doesn't make sense in the context.

I think this section should also have a subsection on Zone focusing. Although zone focusing often is used to describe the “zone” of sharp focus that obtains from a particular focus and f-number, it is also possible to determine the focus and f-number from the required zone, as is discussed in §7.4, Focus and f-number. I'll see what I can put together if no one has a strong objection.

On an unrelated minor issue: I propose that we replace “formulae” with “formulas” throughout to make the article less pretentiously academic. JeffConrad (talk) 02:20, 12 January 2008 (UTC)

[edit] New lead section 11 February 2008

I've made yet another attempt at a lead section; until we get this cleaned up, I don't think the article is a candidate for much of anything. I think there's still plenty of room for improvement, but hopefully this latest attempt will help get the process off dead center JeffConrad (talk) 01:06, 12 February 2008 (UTC)

[edit] DOF and Lens Aberrations

I'm not an expert on optics, but I do wonder what is the cause of DOF. My current understanding is that spherical lenses have a type of aberration that causes limited DOF. Perhaps someone who is more knowledgable than I can comment and make appropriate links to the page on lenses, which has illustrations of various types of aberrations. 129.176.151.7 (talk) 14:54, 21 March 2008 (UTC)

The usual approach to DOF ignores both diffraction and aberrations, and just assumes a perfectly converging cone of rays. Put another way, the only aberration considered is the defocus aberration. It's a simple geometry problem then to find out how much the rays are spread for objects at different distances, and to compute the range of distances that keeps that spread within a selected bound. That's all there is to it. Dicklyon (talk) 01:49, 27 March 2008 (UTC)
As Dicklyon says, it's not caused by any "aberrations". It's just a result of geometric optics. If you have a pinhole camera with a big pinhole, it'll make blurry pictures (blurred by the shape of the pinhole). If you add a lens, you'll get a sharp image if it's in focus but you'll go back to the same blur shape if it's out of focus. —Ben FrantzDale (talk) 02:05, 27 March 2008 (UTC)

[edit] Betacommand edit of 26 March 2008

I don't consider the Cambridge in Colour tutorial link spam. It's been in this article for quite some, and none of the substantive contributors seems to have had an issue with it. Unless someone has an objection, I think we should restore the link. JeffConrad (talk) 21:48, 26 March 2008 (UTC)

that site in question has been repeatedly mass added by the webmaster and others working for that company since 2006. Recently the spammer has been active again. due to the aggressive and forceful method of the spammer their links have been removed countless times. βcommand 21:52, 26 March 2008 (UTC)
I agree. I have removed their links from numerous articles myself; it seems too spammy. Dicklyon (talk) 22:24, 26 March 2008 (UTC)

[edit] 35mm MP area

For a 35 mm motion picture, the image area on the negative is roughly 22 mm by 16 mm (0.87 in by 0.63 in).

That's the camera aperture for Academy (1.37:1). While that may indeed by the image area on the negative, almost no one shoots with that as their intended viewing aperture (or aspect ratio). Is this really what DOF calculations are based on? [Also, it seems weird to give apertures to only two decimals, since normally they are given to three]. Thanks. jhawkinson (talk) 21:44, 3 May 2008 (UTC)

For DOF calculations, you don't need much accuracy in the format size or CoC. If the standard frame is cropped a bit, it doesn't change the format size or CoC criterion or DOF enough to be concerned about. Dicklyon (talk) 23:03, 3 May 2008 (UTC)
The format may have some marginal effect on the DoF mainly because of the method of projection and the fact that most motion picture theaters use a constant screen height and adjust the width of the sides. This means that an Academy ratio film uses the least screen area but almost all of the frame area, while a 1.85 film will crop a significant portion of the frame in the projector, but require a much larger screen area. Therefore, the magnification of a frame will vary within an identical venue depending on which format is being projected. The magnification effect will also be determined by the absolute size of the screen and the distance of the viewer, though, so I would hazard to guess that the CoC number for motion pictures is determined to tolerances able to accommodate many of these parameters where probable. Girolamo Savonarola (talk) 06:09, 4 May 2008 (UTC)

[edit] Effect of lens aperture: entrance and exit pupils

I'm all for being strictly correct, but I think we'll simply confuse most readers if we discuss pupils at this point. Except for extreme closeups with highly asymmetrical lenses, the difference in pupil sizes/locations has almost no effect, and I think we quite reasonably confined discussion of pupillary magnification to a separate subsection. JeffConrad (talk) 00:47, 8 June 2008 (UTC)

I agree. I also left you a comment on your talk page before I noticed that you commented here. Dicklyon (talk) 01:05, 8 June 2008 (UTC)


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