| Date: Wed, 21 Feb 2001 14:54:15 -0500
| From: "Elliott, Rolland" <[EMAIL PROTECTED]>
| Subject: Re: EOS Do Canon IS lenses have problems with Infrared film?
<snip>
| And as far as AF is concerned the red filter will shift the focus somewhat
| to compensate for the focus. Thus shooting at f/5.6 and using AF basically
| solves the focus correction issue.
<snip>
So if I understand the issue correctly, one of the inherent properties of a
typical, non-exotic lens is that light wavelengths are each brought to
slightly different focus. Blue comes in at one point, green comes in at a
slightly different point, red a little further from green, and I now suppose
IR comes in a little further from red, which is why we have those IR marks
on focus scales. The effect ("diffraction," right?) is more pronounced with
telephoto lenses, which have IR marks further away from the regular focus
mark than short lenses do. So what you're saying is that the red filter
will prevent the camera from focusing on blue and green wavelengths and
shift its focus more toward the red end of the spectrum, correct? And then
the DOF at 5.6 is deep enough to accomodate the IR wavelength focus?
One other question -- why is it so important to camera manufacturers to
prevent their AE cameras from metering IR light? If the amount of IR light
in a typical nature scene is approximately proportional to the amount of
visible light, why is it such an issue? Are there special conditions that
have IR light in wildly varying proportions? Do these internal AE filters
block UV light too?
Thanks for an interesting post. I haven't played with IR film in ten years
but now my curiousity is piqued again. :)
Steve Taylor
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