Originally posted by Marc Sabatella The question does not make sense as asked - angles are not measured in mm. It sounds like you are simply asking how the crop factor is applied, and the answer is, the word "factor" is taken literally. That is, you multiply. So a 50mm lens on APS-C format will have the same field of view of as a 75mm lens on 135 format. If you literally want to know the angle - measured in degrees, not in mm - there is indeed a formula for that, but I'd have to Google it...
Correct,
what I tried to ask is for
Lens Equivalence / Diffraction Calculator
which the next poster has send a link to.
Thank you all very much.