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 |
|---|---|---|---|---|---|
| Pentax Q mount | 9.2 mm | No reflex mirror | 2011- | 4 | 2 |
| D mount (8mm movie cameras) | 12.29 mm | ||||
| Nikon 1-mount | 17 mm | No reflex mirror | 2011– | ||
| C-mount (Bolex, Eclair and Bell & Howell) | 17.52 mm | ||||
| Fujifilm X-mount | 17.7 mm | No reflex mirror | 2012– | 3 | 0 |
| Canon EF-M mount | 18 mm | No reflex mirror | 2012– | 1 | 1 |
| Sony E-mount | 18 mm | No reflex mirror | 2010– | 4+2 | 4 |
| 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 |
| 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 | 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 | ||
| Olympus Four Thirds System | 38.67 mm | ||||
| Konica AR | 40.70 mm | 1965–1987 | |||
| Canon FL | 42.00 mm | 1964–1971 | |||
| Canon FD mount | 42.00 mm | 1971–1990 | |||
| Start (Soviet SLR) | 42.00 mm | 1958–1964 | |||
| Minolta SR-mount | 43.50 mm | 1958–2001 | |||
| Fujica X bayonet | 43.50 mm | ||||
| Canon EF mount | 44.00 mm | 1987– | 47 | 64 | |
| Canon EF-S mount | 44.00 mm | more mirror clearance allows elements closer to the sensor; | 2003– | 1 | 8 |
| Praktica B mount | 44.00 mm | ||||
| Sigma's SA mount | 44.00 mm | 1992– | |||
| Minolta/Konica Minolta/Sony A-mount | 44.50 mm | 1985– | 61+10 | 73 | |
| 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 | ||||
| Nikon F-mount | 46.50 mm | 1959- | |||
| Leica R mount | 47.00 mm | 1964–2009 | |||
| Sony B4 mount | 48.00 mm | ||||
| Arri STD | 52.00 mm | ||||
| Arri B | 52.00 mm | ||||
| Arri PL | 52.00 mm | ||||
| T mount | 55.00 mm | ||||
| OCT-19 | 61.00 mm | ||||
| Mamiya 645 | 63.30 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.
Famous quotes containing the word distance:
“It is the simplest relation of phenomena, and describes the commonest sensations with more truth than science does, and the latter at a distance slowly mimics its style and methods.”
—Henry David Thoreau (1817–1862)