Vignetting with IR film

Last weekend I shot a roll of EFKE IR820 using a Nikkormat with several different lenses. A 720nm IR filter was used for all shots. It looks like the shots made with both the 35mm and 28mm PC Shift lenses showed pronounced vignetting when the lenses were shifted to the maximum. I have never seen this with other films, and I assume that it is a function of the fact that IR light focuses differently than full spectrum I did make the slight focus correction for IR indicated on the lens barrel, but that doesn't seem to matter.

Has anyone else experienced this phenomenon, or have some knowledge of it?

Thanks

Any chance it was the filter? (By virtue of extending the lens barrel a little bit)

Duncan

I can guarantee it was the filter ihat caused it. I always use oversize filters with wide lenses.

If you print yourself, then it wouldn't be a problem.

Jeff

Thanks to all of you for your thoughts about this. I remain mystified since I always use both these lenses with filters in place (Skylite, Yellow or Orange) with standard B&W film,and I have not experienced the vignetting problem that I had with the IR filter and film. A more "scientific" set of exposures with and without filter may help to eliminate that variable.