Hi Peter,
You know that I have shot quite some 4*5 IR sheet film, and I never
correct focus, with no ill effects appearently. There was a thread on
this lately on APUG.
Offcourse stopping down will help, but be aware that longer wavelengths
diffract more, so in theory for the IR region (from 700nm (red) and
beyond) you can stop down less than for the visible region.
Another worthwhile point raised in the discussion: all 4*5 sheet film
nowadays is realy extended red/IR sensitive pan film, so you'll register
both visible light and IR light (depending on the filter used) and
visible light obviously does not need focus correction.
The only exception is Kodak HIE film, which is a true IR film..perhaps
you have still sheets left of this film?
Good luck,
Cor
