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For an
interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), back focus
or register, depending on the usage and source) of a
lens mount
A lens mount is an interface – mechanical and often also electrical – between a photographic camera body and a lens. It is a feature of camera systems where the body allows interchangeable lenses, most usually the rangefinder camera, singl ...
system is the distance from the mounting
flange
A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of an iron beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer of ...
(the interlocking metal rings on the
camera
A camera is an Optics, optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), ...
and the rear of the
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
) to the
film
A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
or
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curr ...
plane. This value is different for different camera systems. The range of this distance, which will render an image clearly in
focus
Focus, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
within all
focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foca ...
s, is usually measured to a precision of hundredths of millimetres, and is not to be confused with depth of field.
Lenses can be adapted from one
mount (and respective FFD) to another. FFD determines whether
infinity focus
In optics and photography, infinity focus is the state where a lens or other optical system forms an image of an object an infinite distance away. This corresponds to the point of focus for parallel rays. The image is formed at the focal point ...
can be accomplished with a simple non-optical adapter. Optics to correct for distance introduce more cost and can lower image quality, so non-optical lens adapters are preferred. A simple non-optical adapter holds the longer FFD lens the appropriate additional distance away from the sensor or film on the shorter FFD camera. A ''camera body'' with a ''shorter'' FFD can accept a larger number of lenses (those with a longer FFD) by using a simple adapter. A ''lens'' with a ''longer'' FFD can be more readily adapted to a larger number of camera bodies (those with a shorter FFD). If the difference is small, other factors such as the sizes and positions of the mounting flanges will influence whether a lens can be adapted without optics.
Standard mounts
Typically, camera bodies with shorter flange focal distance can be adapted more readily to lenses with longer flange focal distance.
Precision
Flange focal distance is one of the most important variables in a
system camera
A system camera or camera body is a camera with interchangeable components that constitutes the core of a system. Early representatives include Leica I Schraubgewinde (1930), Exakta (1936) and the Nikon F (1959). System cameras are often sin ...
, 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 camera
A movie camera (also known as a film camera and cine-camera) is a type of photographic camera that rapidly takes a sequence of photographs, either on an image sensor or onto film stock, in order to produce a moving image to project onto a movie s ...
s are rigorously tested by rental houses regularly to ensure the distance is properly calibrated. Any discrepancies between eye focus and measured focus that 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 ground glass (or both) that are misset.
Film use
Due to research on optimal flange focal distance settings, it is currently considered better 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 ground glass, whereas the actual flange focal depth to the aperture plate will in fact be ~0.02 mm less.
See also
*
List of lens mounts
A lens mount is an interface – mechanical and often also electrical – between a photographic camera body and a lens. It is a feature of camera systems where the body allows interchangeable lenses, most usually the rangefinder camera, singl ...
References
* Markerink, Willem-Jan.
Camera Mounts & Registers{{Webarchive, url=https://web.archive.org/web/20071124034629/http://www.a1.nl/phomepag/markerink/mounts.htm , date=2007-11-24 ''. Retrieved on November 6, 2005.
Photography equipment