I have a Vivitar 28~85mm 1:2.8 Zoom Telephoto Nikon AI Lens for my 5D and I'm getting vignetting....can someone tell me why this is?

I don't know anything about that specific lens, but I do know that the reason why the camera is not correcting for it is because Canon disables auto correct of vignetting for its Canon glass only.

The 5D is full frame, so most wider lenses will show some vignetting. The supposed "peripheral illumination correction" isn't doing much for me. Anyone else finding that this works.

My Canon 16-35 show a fairly strong vignetting at 16mm; of course zooming to 20mm or more takes care of it. The 24 1.4 also vignettes a bit.

Aside from zooming in, stopping down your lens will also likely reduce the vignetting.

my 17-40L shows no vignetting, I don't know about other wides as I do not have any.

Thanks Eric for the info.......I'm very new to the whole dslr thing......I played around with it a little and uped the iso and shutter and I see no vignetting.........I'll definitely need to use neutral density filters when using this lens outside.