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Author Topic: What is a back focal length?
Mitchell Cope
Master Film Handler

Posts: 256
From: Overland Park, KS, United States
Registered: Jun 99


 - posted 03-29-2001 07:07 PM      Profile for Mitchell Cope   Email Mitchell Cope   Send New Private Message       Edit/Delete Post 
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?

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Mark Ogden
Jedi Master Film Handler

Posts: 943
From: Little Falls, N.J.
Registered: Jun 99


 - posted 03-29-2001 09:45 PM      Profile for Mark Ogden   Email Mark Ogden   Send New Private Message       Edit/Delete Post 
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.


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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99


 - posted 03-29-2001 11:25 PM      Profile for Gordon McLeod   Email Gordon McLeod   Send New Private Message       Edit/Delete Post 
Usually it is short focal lenghts that require the lens moved forward to open the gate. Especially in century C

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000


 - posted 03-30-2001 08:42 AM      Profile for Pat Moore   Email Pat Moore   Send New Private Message       Edit/Delete Post 
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.

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