[Leica] So, wouldn't a 50mm Summicron LTM or 35mm f2 ASPH LTM be the best enlarging lens on earth?

[benedenia at gmail.com (Marty Deveney)]

> Does an apo lens bring all three primary colors to the same focus point, 
> as opposed to just two out of three for a non-apo lens?

The term is not strictly used correctly by manufacturers, but a truly
apochromatic lens brings all three colours to the same focus point.
Most manufacturers use it to mean "very well colour corrected".  Even
the Leica APO lenses show some chromatic aberration if you push them
hard enough.

> If that is the difference, why would an apo lens offer any advantage for 
> B&W printing?

It affects sharpness; the difference in colour focus makes edges
blurrier, because the light that is projected when you print B&W is
not a single wavelength.  So it matters.

It also doesn't hurt that APO lense stend to be better in other areas
of performance.  The Apo-El-Nikkors are visibly better than anything
else I have used, including the Apo Rodagons and Scheider Apos and the
Zeiss S-Orthoplanars.  But you need to use extremely well adapted
technique to get the best out of these lenses.  Ctein has a free ebook
that includes a chapter on getting the most out of your darkroom:
http://ctein.com/PostExposure2ndIllustrated.pdf

This still feels a bit like discussing stalking techniques for the
next mammoth hunt.

Marty