focal length doesn't physically change, it is a equivalent focal length because the sensor is smaller..
I.e: a 50mm lens on 35mm/full frame DSLR is 50mm..
a 50mm lens on a APS size sensor will have the equivalent FOV of a 80mm lens, on a CANON.. Or a 75mm lens if you are shooting NIKON..
its the same deal with film..
APS DSLR standard lens is 35mm
35mm standard lens is 50mm
6x6cm standard lens is 80mm
4x5 inch standard lens is 150mm
all have the approximate FOV of about 45-50 degrees which is what is 'supposed' to be the closest resemblance to what the human eye sees.. hence standard lens
