In article <4blqlt$gmq@rebecca.albany.edu>, dr8192@CNSVAX.ALBANY.EDU says...
> I use a Sigma 14 not only for effect but for neccesity
> in tight spaces, such as vehicle interiors or crowds.
> The lens has the unavoidable lesser illumination toward
> the edges -- there is no beating the cosine^4 law of the
> angle of incidnece of projected light, and the variation
> in angle of projection from center to edge on a 14mm is
> quite extreme. (rest deleted)

Actually, there are ways to cheat the cosine law (try a 15mm
Nikkor for an example of a super wide with VERY even illumination):
The retrofocus design itself (necessary to allow clearance for
the SLR mirror) moves the lens away from the film, making angular
distance variations between the lens and the film much less than they would be with standard lens design. In addition, the diaphram off
center angle can be made to act as if it were larger than it is (look straight into the 15mm Nikkor [and many other wides], then turn it
to look in from the edge - the diaphram will appear much larger when
viewed from the edge). Having enough glass up front and at the rear
so that the diaphram is not partially obscured at the edge of coverage
also helps.
Hope this helps.