Does anyone have a good explanation of why macro lenses with internal focusing lose focal length at minimum focus? I know it has something to do with how the lens elements are shifting, and possibly even the aperture assembly moving. Can't find any resource on the web and my "Goolge Fu" is usually pretty good.
There is an explanation in the Canon Eyes of EOS book. Internal focus lenses shorten the lens focal length moving the exit pupil away from the sensor making the lens focus closer. This is obvious on macro lenses due to the higher magnification compared to general lenses.
A side effect of this is the lens pupillary magnification changes slightly making a second order change in the theoretical light loss and depth of field for a given set T-stop.