Formula for using a front cell only

[Jonathan Brewer]

Thank you Christopher, I appreciate it, I'll measure it this way and see what I get.

[Ernest Purdum]

Will the front cell screw on in place of the rear cell? This arrangement is usually considered preferable for distortion reasons. Also, with the lens in this position, the iris opening divided into the focal length gives you the f stop accdurately.

[Ted Harris]

Jonathon, FWIW while I can't give you an exact formula I can tell you that I have shot with the front cell only of an Apo Symmar 210 ... did it by accident while shooting formal shots for a wedding ... too long a story for here and yo can find it in the archives. Bottom line is that it appeared to be somewhere in the 300-360 range or a bit longer and that the images came out fine.