What is the effect and importance of the back focal length of a lens?

For instance, if I look at these two lenses by Ricoh, they have the same focal length and aperture, but their back focal lengths differ.

What difference does that make, in layman's terms or in practice?

http://industry.ricoh.com/en/fa_security/lens/pdf/2M/FL-CC1614A-2M.pdf http://industry.ricoh.com/en/fa_security/lens/pdf/2M/FL-CC1614-2M.pdf

2 Answers 2


They are referring to the amount of clearance between the lens rear element and the sensor. The C mount flange focal distance is 17.52mm so both of these lenses have rear elements that stick into the camera body.

This fact is most relevant to SLR cameras that have a mirror which moves out of the way when a photo is taken, a lens that sticks into the camera body could foul the mirror. The C mount is exclusively (to my knowledge) used with cine cameras, microscopes and industrial/surveillance cameras which have no reflex mirror.

Having a lens that is heavily recessed into the camera body can make changing lenses slightly more difficult, but other than that the rear focal length is largely of academic interest as far as I can tell.


From what I know back focal length depend of the optical construction of the lens. And it is important for mirror cameras. For example you can't mount EF-S lens on fullframe camera because last element will interfere with the mirror (which is bigger than APC cameras).

P.S. Above abbreviations and explanation are related to Canon cameras and lens, but IMHO the principle is the same

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