Re: [Leica] apo definitions

[Ken Iisaka <kiisaka@pacbell.net>]

> If we assume extremely small residuals of the secondary spectrum,
> then only reprolenses, microscopes and some telescopes are corrected
> aprochromatically. So "Nikon Special Optics" is right in asserting "
> no current consumer enlarging lenses are true apochromats". If they
> interpret "true" in the sense as discussed above their statement is
> correct but also a half truth! They should have added; " and so are
> most if not all of the consumer photographic lenses". But then Nikon
> has photographic lenses  with the designation 'APO" and no enlarging
> lenses with that sticker?

There are two large format lenses designated as "APO" as well as a 105mm
lens for 35mm photography that is corrected for four wave lengths.

The two of the former are perfectly symmetric, designed for 1:1
reproduction, and thus highly corrected, as the designer only has to
deal with a more limited number of abberations.  Most of the residual
abberations are a result of errors in manufacturing tolerence than the
design.

The latter is the seldom-seen UV-Nikkor 105mm 1:4.5 which is corrected
for wavelengths rom 650nm or so down to 300nm or so (I don't recall the
exact figures, I'm afraid.)  This lens is rather interesting because the
infrared compensation is exactly the opposite of normal lenses: to focus
at infinity, you turn the focussing ring just a bit beyond infinity.

Nikon is still one of few companies which is rather strict about how
they use certain terms.  Recall that Nikon stubbornly calls its close-up
lenses, Micro-Nikkors and Ultra-Micro-Nikkors.

Speaking of APO lenses, multicontrast b/w papers benefit from a more
effectively chromatically corrected lenses, as they are sensitive to a
broader range of wavelengths than conventional graded papers.