----- Original Message -----
From: Shel Belinkoff <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Wednesday, November 21, 2001 12:57 PM
Subject: APO Lenses (was Re: Layers, Sharp Focus, and New-Fangled Color
Film)
> Now this I'd like to hear more about. Can you elaborate on this point,
> specifically, how do manufacturers get away with labeling a lens as
> Apochromatic when it isn't? Is the term "APO" used to imply that a lens
> is apochromatic, when, in fact, it's just a model name given to the
> lens? Which lenses that are designated as "APO" lenses are not, in
> fact, apochromatic? Which are? Just a few examples would be
> appreciated.
The questions that arise are which three wavelengths of light, to what
degree do they converge, at what fstop and focus distance do they qualify,
and how do you measure this all crop up when you ask the above questions.
There isn't (AFIK) any standard for judging what apo correction actually
means. Therefore you end up seeing big difference between a Zeiss APO-Tessar
for the Hasselblad, APO Artar lenses for large format (only judged apo at
1:1 magnification), and a Sigma APO lens (God only knows what they use to
judge). The best advise is to ignore any APO designation that isn't
carefully explained, or better yet just ignore the APO designation all
together. In reality, there are very few types of photography that have to
have that kind of correction...
Isaac
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