Flange focal distance

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Not to be confused with front focal distance.
Image of Flange focal Length (2 types camera)
Diagram Illustrating the Flange Focal length of an SLR–type and a mirrorless–type camera

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), or register, depending on the usage and source) of a lens mount system 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 (not to be confused with the similarly named depth of field).

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, giving 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. Lens adapters are generally easier to make when the camera body has a large lens mount.

Standard mounts include:

Mount Flange
focal distance
Notes In production Number of
prime lenses
produced
Number of
zoom lenses
produced
Number of
wide/tele converters
produced
Number of
mount converters
produced
Pentax Q-mount 9.2 mm[1] No reflex mirror 2011- 5 3 1
D-mount (8mm movie cameras) 12.29 mm
CS-mount (surveillance cameras) 12.5 mm
Nikon 1-mount 17 mm[2] No reflex mirror 2011– 1
C-mount (Bolex, Eclair and Bell & Howell) 17.52 mm
Fujifilm X-mount 17.7 mm No reflex mirror 2012– 10 6
Canon EF-M-mount 18 mm No reflex mirror 2012– 1 3 1
Sony E-mount 18 mm No reflex mirror 2010– 6+5 10+6 2+2 2+2
Sony FZ-mount 18 mm No reflex mirror 2011– 0 2
Micro Four Thirds System 19.25 mm No reflex mirror 2008– 22 17
Samsung NX-mount 25.50 mm No reflex mirror 2010– 5 4
Pentax Auto 110 27 mm 1978-1985 5 1 1
RED ONE interchangeable mount 27.30 mm
Leica M-mount 27.80 mm No reflex mirror 1954–
M39 (Leica) Screwmount 28.80 mm No reflex mirror
Contax G-mount 29.00 mm No reflex mirror 1994–2005 6 1
Olympus PEN F 28.95 mm 1963-1972 16 2
Contax RF-mount 34.85 mm No reflex mirror
Nikon S-mount 34.85 mm No reflex mirror 1947–2005
Standard B4 1/2"-mount 35.74 mm
Sony B4 1/2"-mount 38 mm
Olympus Four Thirds System 38.67 mm 2003- 12 30 2
Konica AR-mount 40.50 mm 1965–1988 48 13 1 4
Canon FL-mount 42.00 mm 1964–1971
Canon FD-mount 42.00 mm 1971–1990 107 34
Start (Soviet SLR) 42.00 mm 1958–1964
Minolta SR-mount 43.50 mm 1958–2001
Fujica X-mount 43.50 mm
Canon EF-mount 44.00 mm 1987– 47 64
Canon EF-S-mount 44.00 mm 2003– 1 8
Praktica B-mount 44.00 mm
Sigma SA-mount 44.00 mm 1992–
Minolta/Konica Minolta/Sony A-mount 44.50 mm 1985– 43+20 54+26 8+2 2
Exakta 44.7 mm[3] 1936–1969
Pentax K-mount 45.46 mm 1975– 131 77
M42 45.46 mm 1949–
Contax C/Y-mount 45.50 mm 1974–2005
Olympus OM-mount 46.00 mm 1972-2002 38 14
Nikon F-mount 46.50 mm 1959-
Leica R-mount 47.00 mm 1964–2009
Sony B4 2/3"-mount 48.00 mm
Contax N-mount 48.00 mm
Arri STD 52.00 mm
Arri B 52.00 mm
Arri PL 52.00 mm
T-mount 55.00 mm
Panavision PV-mount 57.15 mm 35mm & 16mm 1972-
OCT-19 61.00 mm
Mamiya 645 63.30 mm
Contax 645 64.00 mm
Pentax 645 70.87 mm 1984-
Hasselblad 2000 & 500 74.90 mm xxxx-
Pentax 6x7 84.95 mm 1984-
Pentaflex (16mm cameras) 44 mm

Flange focal distance is one of the most important variables in a system 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 Arri PL mount with an FFD of 52.00 mm. The Russian OCT-19 has an FFD of 61.00 mm. C-mount (Bolex, eclair and Bell & Howell) has 17.52 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. Discrepancies in flange focal distance are much easier to identify with wide lenses than with telephoto lenses, due to wide lenses having smaller depth of focus. (Not to be confused with depth of field.)

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 groundglass, whereas the actual flange focal depth to the aperture plate will in fact be ~0.02 mm less.

See also[edit]

References[edit]

  1. ^ [1] pentax.jp
  2. ^ [2]
  3. ^ [3] Wrotniak, J. Andrzej