Originally posted by Digitalis Let me put it this way, using a smaller sensor is not like cutting cylinders* through the lens elements, throwing away the outer perimeter (annulus) of all the glass, and just using "the center of the lens". In fact, the full diameter of the entrance pupil of the lens is always employed, regardless of how tight a sensor crop you take, because the entrance pupil diameter (D) defines the focal ratio (f) in conjunction with the Focal length F (f = F/D). Since cropping doesn't change focal length, and doesn't change the image brightness, it cannot change the entrance pupil size either. So in order to be effective on this lens, the hood should be bigger IMO otherwise even with the hood attached flare will be problematic on both DX and FX format.
* in actuality it is more like a cone going from the annulus of the first element and receding in diameter towards the focal plane. Though this is only a model for when the lens is used at its widest aperture, things change once you start stopping the lens down.
You're thinking way too hard. This has nothing to do with entrance pupils or anything.
Angle of view simply means that incident light within a certain angle of the axis of the lens will be seen by the sensor. Anything occupying that space will be seen by the sensor. A hood optimized for APS-C will be deeper, because it can occupy space that would otherwise be seen by a FF sensor. An FF optimized hood
must be shallower than an APS-C optimized hood.
This is extremely easy to prove. Put virtually any APS-C zoom lens with a hood on a FF camera at the widest focal length, and you can actually see the hood in the viewfinder, because it is deep enough to appear in the frame. I just did it right now.