Winsor writes:
So if I understand you correctly, a well designed 300/f4.5 with a little
low dispersion glass will correct for chromatic aberration as well a faster
APO design with ultra low dispersion glass. The design of the lens, like
other design, is using materials appropriate to the task.
===============================
Not quite what I said. at f/5.6 a 300mm lens doesn't need an apochromatic
design. I didn't say exactly where it becomes critical, but given that
Olympus includes some low dispersion elements in the later 300/4.5 designs
(it wasn't in the early ones) they obviously think it needs it, so I'll
assume it does. For sure, a 300/4 does.
You last statement I agree with and would mention that it really isn't
necessary to get hung up on APO/ED glass. This only has to do with
chromatic aberration and not other aspects of lens sharpness or contrast.
All that matters is that you don't see color fringing in your images.
If you have a sharp lens without color fringing, rest easy and don't
worry about how the lens designers achieved the result.
and Gary writes:
================
Yup. I'd say you have it down. For a 300mm, an f/5.6 design doesn't need
expensive glass, so you could have an APO design at a relatively low cost.
================
yes, but it wouldn't be an APO design, rather an achromatic design.
Before low dispersion glass was used there was a technique of
using achromatic doublets. These are two elements glued together
into a single group. The second element corrects the chromatic
aberration in the first (for two colors). apochromatic designs
correct 3 color for chromatic aberration and use low dispersion glass.
There is alot of marketing hooey surrounding apo lenses for sure.
I think the cheaper Sigma lenses labelled APO are not truly apochromatic
designs, but just lenses that had some low dispersion glass mixed
in with conventional glass in the melt. Whatever they are, I
once owned the Sigma 75-300/4-5.6 APO and was not overly impressed
with the optical quality.
j. albert
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