Quote Originally Posted by SMBooth View Post
I would I be right in saying that just because a faster lens has a greater image circle, the vignetting or edge falloff start around the same distanse from the centre of the lens, or does a F4.5 with a 235mm IC show less fall off on a 6x17 image then a f8 with 216mm IC because the IC is bigger and the falloff is further out.
Lenses have a natural cos^3 or cos^4 falloff with respect to angle and because a given film size subtends a greater angle with a shorter focal length, there will be more falloff than with a longer focal length. Different lens designs lead to different amounts of this natural falloff.

It's got nothing to do with coverage, which is generally where mechanical vignetting kicks in and gives you a distinct image circle. And likewise, a faster lens does not necessarily have greater coverage at all - tele lenses (where the rear nodal point is well forward of the rear of the lens) generally have less coverage than you might expect from their focal length.