[Leica] reply: APO

[Erwin Puts <imxputs@knoware.nl>]

<Do you know, off hand, what the wavelengths for C, D, and F are?

C=6563
D=5890
F=4861
in Angstrom units= 10 power -7 mm

<Are you saying Leica doesn't demand apochromatic correction across the
<image field, edge to edge? I seem to recall Leica stating that very thing.

If a lens system is APO corrected it is apo corrected all over the field.
But some other aberations  can diminish the image quality in the outer
zones. If Eric implies that a lens can be fully apochromatically corrected
in the centre and be non- or partially-apochromatically corrected in the
outer zones, I at least must pass.

<I seem to recall Leica stating that very thing.

I have no knowledge of this particular claim by Leica.

<And your comment about glasses having apochromatic characteristics is
<misleading, no? No particular glass could be so described. It's a
<combination of glass elements that brings about apochromatic correction. Do
<you mean some glasses have high refractive index and anomalous dispersion
<characteristics which makes such correction possible?

Yes my statement is misleading. Thanks for pointing your finger at it. To
create apochromatic correction you need three glass types some of them of
anomalous dispersion character. There is no APO glass. ( I said that myself
in an earlier post, but I was to hasty in my wording). What I meant to say
is that some glasses have characteristics that can advantegeously be used
in combination with others to create APO correction.