[Leica] APO for B & W?

[Mike Johnston <michaeljohnston@ameritech.net>]

Doug Herr>>>
Unless your light source is a single wavelength, light is color whether
the film
or paper records separate wavelengths or records intensity (as in B &
W).  You
want all wavelengths to focus at the same place otherwise you get a blur
instead
of a point.  An APO lens is defines as one where 3 wavelengths (I forget
which 3)
at various points in the spectrum focus at the same point.  Non-APO
lenses don't
nessesarily do this.<<<


This is true. The closer any lens comes to making images of identical
size from every color of light, the better the lens will be, for color
or black-and-white.

However, since no current "apo" enlarging lenses are truly apochromatic,
it's best to think of "apo" as being synonymous, more or less, with
"deluxe"--what the lensmakers tend to term their best or top-line
lenses. It's a case of "some lenses are more apo than others."

- --Mike