Re: IR Focusing with Canon EOS lenses

["Mark W. Johnson" <markwjohnson@xxxxxxxxxxxxx>]

Infra Red wrote:
> 
> 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? ...
> 
The lens manufacturers are generally making lenses corrected only for
visible light.  Achromatic lenses are corrected for red and blue light
(that is, red and blue light have approximately the same focal length),
and the green light in-between does pretty well under most
circumstances.  But the focal lengths for IR and UV light are poorly
corrected.  This is represented schematically in the diagram below.
       | 
       |            ________
Focal  |       __---        ---__
Length |    _--                  --_
       |   -                        -
       |  -                          -
       | -                            -
       +----------------------------------
             IR   Red  Grn  Blue   UV

Apochromatically corrected lenses use more expensive glasses and more
elements to correct for red, green and blue, flattening the curve to
look schematically more like the following, and thus improving IR focus
shift.
       |
       |                            __
Focal  |           ___         ___-- 
Length |     ___---   ---___--- 
       | __--               
       |                        
       +-------------------------------
            IR   Red  Grn  Blue   UV
However, most apochromatically corrected lenses are long telephotos.

In terms of IR focus shift, zoom lenses are going to be the most
difficult to get a handle on.  There's no reason to believe that a IR
focus correction at 28mm will even be similar to the correction at
105mm.

I think if you have a specific couple of fixed lenses or a particular
zoom lens that you want to use with IR, it is worth doing some carefully
composed test shots with distinct objects at measured close, mid-range,
and near infinity distances, then to shoot the same scene at several
well annotated lens focus settings with the lens wide open.  After
developing the pictures and comparing them with your notes, you should
be able to come up with some pretty good guidelines for using your own
equipment, and when you stop-down the lens, you'll get the assistance of
depth of field.

Mark
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