This is topic What is a back focal length? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Mitchell Cope (Member # 32) on 03-29-2001, 07:07 PM:
I'm looking at some lens specifications. Of course I'm familiar with the term "focal length", but what is a back focal length? How is it important to me? Does it change the distance my lens would normally sit from the film plane?
Posted by Mark Ogden (Member # 43) on 03-29-2001, 09:45 PM:
The back focal length is the distance from the very rear lens element to the film (as opposed to the focal length which is measured to about the center of the lens or so). General rule of thumb: the wider the angle of the lens the larger the back focal length. How importaint is it? If your lens turrent swings out, it's not importaint at all. On the other hand, with a very long lens you would potentially have to nudge the lens forward to open the gate.
Posted by Gordon McLeod (Member # 33) on 03-29-2001, 11:25 PM:
Usually it is short focal lenghts that require the lens moved forward to open the gate. Especially in century C
Posted by Pat Moore (Member # 380) on 03-30-2001, 08:42 AM:
You're right Gordon. Also the earlier Pro35's needed sort focal length lenses moved forward to open the gate.This was mostly a problem with the very "fast" (f1.9 and less) short focal length lenses of the 70's and early 80's. As newer lens designs came around, the back focal length measurement was useful to know so you could anticipate a potential problem. The back focal length of the modern lenses is quite long now so the lens is not right "on top of" the film to be able to fopcus on it and gather all the light.