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 (FEL), or register, depending on the usage and source) of a lens mount

Lens mount

A lens mount is an interface — mechanical and often also electrical — between a photographic camera body and a lens. It is confined to cameras where the body allows interchangeable lenses, most usually the single lens reflex type or any movie camera of 16 mm or higher gauge...

 system is the distance from the mounting flange (the metal ring on the camera and the rear of the lens) to the film

Photographic film

Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of 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

Depth of focus

Depth of focus is a lens optics concept that measures the tolerance of placement of the image plane in relation to the lens...

.

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	5
C-mount (Bolex Bolex Bolex is a Swiss company that manufactures motion picture cameras and lenses, the most notable products of which are in the 16 mm and Super 16 mm formats. The Bolex company was initially founded by Jacques Bogopolsky in 1927. Bolex is derived from his name. He had previously designed cameras for... , Eclair Eclair (camera) Éclair was a film production, film laboratory and movie camera manufacturing company established in Épinay-sur-Seine, France by Charles Jourjon in 1907....  and Bell & Howell)	17.52 mm
Sony E-mount	18 mm	No reflex mirror	2010-	2	2
Micro Four Thirds System Micro Four Thirds system The Micro Four Thirds system is a standard created by Olympus and Panasonic, and announced on August 5, 2008, for mirrorless interchangeable lens digital cameras and camcorders design and development...	20 mm	No reflex mirror	2008-	10	14
Samsung NX mount Samsung NX mount The Samsung NX-mount is the lens mount used on NX series mirrorless interchangeable lens cameras by Samsung. The mount was first implemented in the Samsung NX10, and Samsung initially referred to the NX line as 'hybrid digital cameras', citing their combination of attributes of both DSLR and...	25.50 mm	No reflex mirror
RED ONE interchangeable mount	27.30 mm
Leica M mount Leica M mount The Leica M mount is a camera lens mount introduced in 1954 with the Leica M3, and a range of lenses. It has been on all the Leica M series up to the current film Leica M7 and digital Leica M9....	27.80 mm	No reflex mirror
M39 (Leica) Screwmount M39 lens mount The M39 lens mount is a screw thread mounting system for attaching lenses to 35 mm cameras, primarily rangefinder Leicas. It is also the most common mount for Photographic enlarger lenses....	28.80 mm	No reflex mirror
Contax G Contax G The Contax G1 and Contax G2 are interchangeable-lens cameras sold by Kyocera under the Contax brand in competition with the Leica M7, Voigtlander Bessa R, and Konica Hexar RF. The G1 was introduced in 1994 with the G2 joining it in 1996...	29.00 mm	No reflex mirror
Olympus Four Thirds System Four Thirds System The Four Thirds system is a standard created by Olympus and Kodak for digital single-lens reflex camera design and development.The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different...	38.67 mm
Konica AR	40.70 mm
Canon FL Canon FL Canon FL refers to a lens mount standard for 35mm single-lens reflex cameras from Canon. It was introduced in April 1964 with the Canon FX camera, replacing the previous Canon R mount. It was in turn replaced in 1971 by the Canon FD lens mount...	42.00 mm		1964–1971
Canon FD mount Canon FD The Canon FD lens mount is a physical standard for connecting a photographic lens to a single-lens reflex camera body. The standard was developed by Canon of Japan and was introduced in March 1971 with the Canon F-1 camera. It was the primary Canon SLR lens mounting system until 1987 when the...	42.00 mm		1971–1990
Start (Soviet SLR)	42.00 mm		1958–1964
Minolta SR mount	43.50 mm
Fujica X bayonet Fujica X bayonet The Fujica X mount was a lens mount created by Fujifilm in the late 1970s for the new Fujica SLR lineup : AX-1, AX3, AX-5, STX-1, STX-1N, STX-2. It replaced the old M42 screw mount used on their earlier SLRs....	43.50 mm
Canon EF mount Canon EF lens mount Introduced in 1987, the EF lens mount is the standard lens mount on the Canon EOS family of SLR film and digital cameras. EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens...	44.00 mm		1987–	47	64
Canon EF-S mount Canon EF-S lens mount The EF-S lens mount is a derivative of the EF lens mount created for a subset of Canon digital single-lens reflex cameras with APS-C sized image sensors. It was released in 2003. Cameras supporting the EF-S mount are backward-compatible with the EF lens mount and, as such, have a flange focal...	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 Sigma SA mount The Sigma SA mount is a design of lens mount designed by the Sigma Corporation of Japan for use on their single-lens reflex camera designs. The SA mount uses a bayonet mount which is physically similar to the Pentax K mount but uses a flange focal distance of 44 mm, identical to that of the Canon...	44.00 mm
Minolta/Sony A mount	44.50 mm
Pentax K mount Pentax K mount The Pentax K mount, sometimes referred to as the "PK mount", is a lens mount standard for mounting interchangeable photographic lenses to 35 mm single-lens reflex cameras. It was created by Pentax in 1975, and has been used by all Pentax 35 mm and digital SLRs since...	45.46 mm
M42 M42 lens mount The M42 lens mount is a screw thread mounting standard for attaching lenses to 35 mm cameras, primarily single-lens reflex models. It is more accurately known as the M42 × 1 mm standard, which means that it is a metric screw thread of 42 mm diameter and 1 mm thread pitch...	45.46 mm		1949 -
Contax C/Y Mount	45.50 mm		1974–2005
Olympus OM mount Olympus OM system The Olympus OM System was a line of 35mm single-lens reflex cameras, lenses and accessories sold by Olympus between 1972 and 2002 ....	46.00 mm
Nikon F-mount	46.50 mm
Leica R mount	47.00 mm
Sony B4 mount	48.00 mm
Arri PL Arri PL Arri PL is a lens mount developed by Arri for use with both 16 mm and 35 mm movie cameras. The PL stands for "positive lock". It is the successor mount to the Arri bayonet; however, unlike the bayonet mount, it is incompatible with older Arri-mount lenses, due to the larger diameter...	52.00 mm
T mount	55.00 mm
OCT-19	61.00 mm
Mamiya 654	63.30 mm

Flange focal distance is one of the most important variables in a system camera

System camera

A system camera is a camera with interchangeable components that constitutes the core of a system. Early representatives include Leica I Schraubgewinde , Exakta and the Nikon F...

, 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

Movie camera

The movie camera is a type of photographic camera which takes a rapid sequence of photographs on strips of film which was very popular for private use in the last century until its successor, the video camera, replaced it...

s are rigorously tested by rental houses regularly to ensure the distance is properly calibrated. The most common mount is the Arri PL

Arri PL

Arri PL is a lens mount developed by Arri for use with both 16 mm and 35 mm movie cameras. The PL stands for "positive lock". It is the successor mount to the Arri bayonet; however, unlike the bayonet mount, it is incompatible with older Arri-mount lenses, due to the larger diameter...

 mount with an FFD of 52.00 mm. The Russian OCT-19 has an FFD of 61.00 mm. C-mount (Bolex

Bolex

Bolex is a Swiss company that manufactures motion picture cameras and lenses, the most notable products of which are in the 16 mm and Super 16 mm formats. The Bolex company was initially founded by Jacques Bogopolsky in 1927. Bolex is derived from his name. He had previously designed cameras for...

, eclair

Eclair (camera)

Éclair was a film production, film laboratory and movie camera manufacturing company established in Épinay-sur-Seine, France by Charles Jourjon in 1907....

 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. As per depth of focus

Depth of focus

Depth of focus is a lens optics concept that measures the tolerance of placement of the image plane in relation to the lens...

, discrepancies should be much easier to spot on telephoto lenses than on wider ones.

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.