>>4139133Why do you need to refocus for infra red, but not (deep) visible red?
I shot some Fomapan 400 which just barely juts into IR with an R72 filter which just barely lets very deep red through. The shots I didn't refocus to the red mark on the lens were indeed out of focus, but the shots on which I used a 25A red filter were perfectly sharp. Is there not at least some adjustment needed? My thinking here is there's no sudden change in the physics between visible red and infra red, and very near IR (which is what I captured) should behave about the same as deep visible red.
By way of numbers, deep violet is at 400nm, deep red is 700. That's a 300nm difference, but needs no focus readjustment. But stray into IR (say <725) and suddenly the picture is OOF.
What gives? I must be missing something.
