Flange Focal Distance & What FFD Means for You
|Close-up showing the FFD of a Sony E-Mount camera
Focal Flange Distance/Flange Focal Distance or FFD (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), or simply register, depending on the usage and source) is the distance from the joint of the camera and lens to the digital sensor (or film). This distance is unique in each camera system out there. Depth of Focus, measured in hundredths of millimeters, is the distance between the lens and the image plane that allows for acceptably sharp focus to be achieved. Depth of Focus is also referred to as lens-to-film tolerance. Combining the correct Depth of Focus and FFD allows for infinity focus.
|The Depth of Focus on a Canon EF Mount lens
FFD will also dictate the ability to mount lenses from other camera systems onto your camera body. Generally speaking, cameras with a short flange focal distance can be adapted to a greater diversity of lenses due to the ability to fit an adapter between the lens and camera body. To make sure that your camera permits focus without the use of corrective optical lenses, the FFD must be correct. This means that the thickness of the adapter must be precise to ensure that the FFD for your lens/camera combination is accurate.
Vintage medium format lenses offer excellent opportunities to adapt to modern cameras. These lenses are often relatively inexpensive and simple to adapt due to the long FFD of medium format SLR camera systems. Among modern 35mm SLR systems, the Canon EOS (EF/EF-S) system has the shortest Focal Flange Distance, making it possible to adapt nearly any 35mm or medium format system lens with the appropriate adapter, however there are some exceptions: lenses for 35mm rangefinder cameras and, ironically, lenses for Canon’s other mounts (RF, FL, FD, FDn). Virtually all vintage lenses and DSLR lenses, including those for 35mm rangefinder cameras, can be adapted to Mirrorless Interchangeable lens Cameras (MILCs) thanks to the short FFD of MILCs.
Flange focal distance is one of the most important variables in a camera system, as lens seating errors of as little as 0.01 mm will manifest themselves critically, often resulting in a mismatch of focusing distance in relation to the lens markings and the inability to reach infinity focus. If a lens mount adapter is even a fraction of a millimeter too long the lens will not focus to infinity. To take into account any tolerance discrepancies in manufacturing on both lenses (including third party lenses) and cameras, we specifically make our adapter slightly thinner than the FFD calls for; this ensures that all lenses for one system, regardless of age or manufacturer, will reach infinity focus or slightly beyond when mounted on the camera. While this may be frustrating to some users accustomed to a hard-stop at infinity, it beats the alternative of not reaching infinity focus at all. For this reason, when using a lens mount adapter, it is best to focus visually through the camera rather than relying on the distance markings of the lens to ensure proper focus.
|Illustrating the FFD between Canon EOS (EF/EF-S) and Sony E-Mount camera
systems and how an adapter makes up the needed FFD for the Canon lens
If the difference is small, other factors, such as the diameter of the mounting flanges of the two systems also play a role in the equation. Lens adapters are generally easier to make when the camera body has a large diameter lens mount. This is one of the reasons why Fotodiox does not make a Canon EOS (EF/EF-S) lens to Nikon F camera body lens mount adapter: Canon mounts are 54mm in diameter with a 44mm FFD, Nikon F mounts are 44mm in diameter with a FFD of 46.5mm. This means that for a Canon EOS (EF/EF-S) lens to focus properly on a Nikon F body, the lens would need to be recessed 2.5mm inside the body housing and be 10mm slimmer in diameter to fit inside the mounting flange which physics just doesn’t allow. On top of these issues the recessed portion of the lens would interfere with the mirror mechanism in the camera. Lenses designed for a smaller image sensor may not be adaptable to camera bodies with larger sensors as the image circle may not be large enough to cover the entirety of a larger sensor and will exhibit vignetting. The age of the camera body may limit the availability of adapters; as camera bodies typically reach obsolescence before lenses.
|Adapters come in all shapes and sizes!
Now what if you really want to put a lens with a shorter FFD on a camera with a longer FFD? For example: a M42 screw mount lens is normally adapted to a Nikon F-mount camera with the inability to focus to infinity - as the Nikon bayonet mount FFD is 46.5mm which puts the film plane further away than the M42 screw mount film plane of 45.46mm, this gives a negative -1.04mm of space and leaves absolutely no room for the adapter itself. However, glass adapters are available for some combinations of lenses and cameras, which contain an internal corrective optic (usually 1.4x). This makes the combination mechanically possible, albeit with a small amount of cropping at the edges of the image. The optical components of these adapters refocus the lens to effectively increase the FFD available. This increase in FFD allows Fotodiox to build an adapter that will allow the lens to focus properly on your camera system.
|Lens mount adapter with internal corrective optic
Lastly, we get a lot of questions about making specialty type adapters (e.g. ND Throttle, TLT ROKR) within a single camera ecosystem (e.g. Sony E-mount to Sony E-mount). As the lens and camera are already an exact match for FFD there is simply no room to add an adapter, let alone the mechanics of more complicated adapters such as the examples above.
We hope this helps take some of the mystery our of Lens Adapters! There are several combinations we would love to create, but just aren’t possible due to the FFD. Stay tuned for more fun facts and tutorials from your Fotodiox Crew!
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