The issue here for DSLRs happens both because of the sensitivity
to the angle of incidence of the light AND the fact that SLR
lenses generally are not designed with a huge circle of coverage.
They don't have to - nothing moves on these cameras as compared to,
say, a view camera.
One could probably avoid this problem to a large degree by using
lenses that cover a full sized sensor on a half-frame camera or
even older, manual focus lenses designed for 35mm film on a newer
digibody. The problem there is that Nikon seems to be the only
vendor that has preserved their lens mounts, and even then,
an old AI or AI-S lens does not meter properly on anything but
their highest end digital cameras.
In short, digital makes this a big pain...
