IR Focusing with Canon EOS lenses

[Infra Red <irlist@xxxxxxx>]

I wonder if anyone has any information on which Canon EOS lenses might be more
"IR achromatic" then others, i.e. need less focus shift for IR? I did ask
Canon but they said that that sort of information never leaves the design lab
and isn't available to the public.

Of course I tried using the IR focus markes on lenses like the 28-105, but
still had problems. At 105 the IR focus mark seems pretty good for close
subjects, but at 28 focus seemed way off when using HIE with a 25 red filter,
both at visible light focus AND at the suggested focus shift for 28mm IR. Does
anyone know if the IR focus shift as indicated on the lens is a strong
function of focus distance (I've seen that suggested)?

Just looking at the IR focus marks on the Canon lenses, I can't see that any
of them (17mm to 100mm) seem to be better IR corrected than the others, but
maybe that's not a good way to judge. The longer lenses either lack IR focus
marks or have them only for the wide end of zooms (e.g. the 80-200/2.8L only
shows an IR focus correction at 80mm). Does that mean they don't need focus
shift, or do they just not bother with the markings on the longer lenses?

Bob Atkins

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