This factor affects strongly the depth of field:Code:exit pupil diameter Pupil Factor = ----------------------- entrance pupil diameter

http://toothwalker.org/optics/dofderivation.html

and maybe Airy disk diameter. Most of all in macrophotography.

When a lens is reverted the exit pupil becomes probably the entry pupil, what changes the f-number and i.e. even the hyperfocal distance.

Is it possible to measure it somehow ?

Actually the f-number definition gives the entrance pupuil. But what about the exit pupil ?

Perhaps very approximately????Code:last rear lens diameter P = ------------------------------------------------ focal length / lens brightness (for example 2.8)