Quote Originally Posted by Dan Fromm View Post
David, are you sure its back focus and not focal length? Really, really sure?

If you are, please direct me to a reference.

Cheers,

Dan
I was just glancing through my Kodak Professional Photoguide (3rd edition, 1998) and found an entry on close-up exposure (pp. 46 - 47) which confirms that the usual formula does not work with retrofocus or telephoto lenses. Kodak suggests a formula for these lenses:

Exposure factor = (M/P + 1)

where
M = Magnification
P = Pupillary magnification

P is calculated as rear exit pupil diameter divided by front entrance pupil diameter (so it is specific to a particular lens).

Will send PDF file of these pages to anyone interested.

Regards,

David