[ATM] Minimum Focus Shift with f-number

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

A while back, I gave an all-ECO Petzval design with an
air-spaced N-BK7/N-F2 "primary" and a cemented/oiled
N-BK7/N-BAK4 "secondary".  I recently parameterized it
to get a set of 200 mm aperture designs with f-numbers
ranging from f/7.5 to f/12.5 that each provide a 42 mm
diameter field and appended the information to my
Rogue's gallery:

    http://users.rcn.com/rflrs/roguesgal.htm

Now for the kicker.  During this work, I found an empirical
expression:

          540 + 1.75 * ( f/# - 10 )

for the wavelength (in nm) of the minimum paraxial focus, when
the color is shifted to provide the best Visual Strehl.  I
wonder if this expression. or something like it, would at all
be universal for achromats in general.
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