Flange focal distance

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The flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, or register, depending on the usage and source) of a lens mount for an interchangeable lens camera system is one of its most fundamental attributes. This is the distance from the mounting flange—the metal ring on the camera and the rear of the lens—to the film plane. This value is different for different camera systems. The range of this distance which will render an image clearly in focus within all focal lengths is usually measured in hundredths of millimeters and is known as the depth of focus.

This distance influences whether a lens from one system can be mounted with an adaptor to a camera body of another system. In order to produce an adaptor that permits focus to infinity without corrective optics, the flange-to-film distance the lens is designed for must be greater than that of the camera body it is to be adapted to, to give room for the adaptor. Camera systems with a large flange-to-film distance have lenses that can be widely adapted, while those with a small flange-to-film distance can take adaptors for many types of lenses.

If the difference is small, other factors, such as the diameters of the mounting flanges of the two systems, come into play as well. Camera bodies with wider lens mounts are easier to produce lens adaptors for.

Systems with short flange-to-film distances include Canon's FD (42.00 mm)and EF mounts (44.00 mm), Konica's AR (40.70 mm), and Olympus's Four Thirds system (38.67 mm). These can readily take many other brands of lens.

Systems with larger flange-to-film distances include Nikon's F-mount (46.50 mm), Olympus's OM mount (46.00 mm), and the M42 (45.5 mm) universal thread mount. These lenses can be adapted to many camera bodies.

Flange focal distance is one of the most important variables in a camera, as lens seating errors of as little as 0.01 mm will manifest themselves critically on the imaging plane and focus will not match the lens marks. Professional movie cameras are rigorously tested by rental houses regularly to ensure the distance is properly calibrated. The most common mount is the PL mount with an FFD of 52.00 mm. The Russian OCT-19 has an FFD of 61.00 mm. Any discrepancies between eye focus and measured focus which manifest themselves across a range of distances within a single lens may be collimation error with the lens, but if such discrepancies occur across several lenses, it is more likely to be the flange focal distance or the groundglass (or both) which are mis-set. As per depth of focus, discrepancies should be much easier to spot on wider lenses than on telephoto ones.

Due to research on optimal flange focal distance settings, it is currently considered more optimal for flange focal distance to be set to somewhere within the film's emulsion layer, rather than on the surface of it. Therefore, the nominal flange focal depth will be equivalent to the distance to the groundglass, whereas the actual flange focal depth to the aperture plate will in fact be ~0.02 mm less.