[Leica] Apo defined?

[Mike Johnston <michaeljohnston@ameritech.net>]

>>>Please explain to me what "truly apochromatic" is in relation to an
enlarging lens.<<<


Erwin,
I'm sure there is nothing I can tell you that you don't already know,
but as I understand it, there are two distinct definitions of
"apochromatic": one simply means a lens that has been "corrected" for
three primary spectral colors, and the other, stricter definition is a
lens that gives three images of identical size for three different
spectral lines or regions. IOW, the looser definition implies the
attempt to correct, and the stricter definition implies success in the
attempt. Ray refers to the latter as "true" apochromatic correction. As
you know, there are few camera lenses and some process lenses that are
true apochromats, but not nearly as many as say "Apo" on them. Cf. Ray,
p. 283: "Some lenses are designated 'apochromatic' but are not truly so;
they just have a reduced secondary spectrum, which certainly improves
performance.... The very few true apochromatic lenses used for direct
screening work are rather long focus relative to the format, have
correction extended to 750 nm, and are very expensive."

The intelligence that no current consumer enlarging lenses are true
apochromats comes from Nikon Special Optics.

- --Mike