Sigma vignetting? Aargh!

Shot some HP5(film type doesnt matter, here) with my Minolta 450si and Sigma A IF 28-105 1:3.8-5.6 and found some annoying vignetting on the shots that were backed out to 28mm. Is this a common trait with the lens? I don't remember it happening before...

Big ole nasty donut! It's not a Lomo damn it! What gives?!

I have the Sigma 28-135mm 1:3.8-5.6 macro that also suffers this problem. I have it on a Nikon F-401x. At 38mm it covers no problem, but vingetting starts just below that and gets worse the further you go.

Nuts!