Re: LF infrared focus

[Ed Scott <Dotted.Dog@xxxxxxxxxxxxxxxx>]

When I became aware of this, I thought it might have to do with early
lens designs being poorly corrected for infrared.  I am more inclined to
think the 1/70th of focal length value was just a mistake.

The diffierence between refractive index for Crown glass across the
range of the F to C lines (486 to 656 nm) is 0.007.  The refractive
index difference for Flint glass across the same range would be about
0.01.  The difference in focal length would be unlikely to exceed these
numbers across this range because lens correction would reduce the range
of focal length variability.  Ansel Adam's value of 1/70th represents a
difference in focal length of 0.0143.  If we knew refractive indices in
the IR region it might be possible say with more certainty, but just the
correction over the visable range of 400 to 700 nm should result in some
IR corrrection, it seems unlikely to me that a corrected lens could
exhibit 1/70th focal length difference for the 700 to 900 nm IR region.  

It must be a mistake.  Computers have allowed better lens designs and it
is likely that average IR focal length difference may have been reduced
over the years, but a factor of 5.7 does seem unreasonable.  The 1/4
percent value or 0.0025 factor seems more reasonable in comparison to
change in refractive index across the range of the visible spectrum.

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