[Leica] Leica IR filter

[henningw at archiphoto.com (Henning Wulff)]

>The filter controls what wavelengths get through, of course.   The 
>filter therefore can be ground to compensate the focus point for 
>those wavelengths. What the film records should not be a factor, 
>since it can only record what gets through to it.
>
>Bob
>

All filters let through a large range of wavelengths; most IR filters 
pass wavelengths well beyond 900nm while few films are sensitive in 
that region. Therefore compensation differences due to the emulsion 
matter.

Since the main issue for the IR filters is the low cutoff to match it 
to subject matter, film sensitivity and thus exposure time, the upper 
end is usually left free to take advantage of whatever sensitivity 
the film may have.

-- 
    *            Henning J. Wulff
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