Maybe my mind has failed me again but the shape of the hood doesn't
matter. All that matters is that light rays at an angle greater from the
optical axis than will form part of the image be blocked while none of
light rays forming the image be blocked. Whether its rectangular or
tulip shaped the same thing can be accomplished.
Think of parallel light rays striking the entire front lens surface at
some angle. The shape of the lens bends all of these rays so that they
strike the same point on the image plane. This is why lenses don't have
to be rectangular. If the angle is different they will strike a
different point.
If the lens hood blocks none of the image forming rays then some
non-image forming rays will strike the far side of the front lens
surface and bounce around until they are absorbed. You can't block all
undesired rays without also blocking some desired rays. So ideally the
hood should block none of the desired and as many of the undesired as
possible. If the blocking surface is laterally further from the optical
axis it has to be further forward to block rays coming from the same
angle. The only advantage of a rectangular hood is that all of the
forward most allowable blocking is in a plane perpendicular to the
optical axis.
Too bad there aren't sliding non-rotating tulip shades built into
lenses.
-jeff
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