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[OM] 300/4.5

Subject: [OM] 300/4.5
From: Joseph Albert <albert@xxxxxxxxxx>
Date: Sat, 26 Sep 1998 03:36:08 -0700 (PDT)
========================
> I thought the 300/4.5 was built with low dispersion glass which is what APO
>  is all about, making all the colors focus in the same place.

Low dispersion glass is not particularily noteworthy or special.
...
partial dispersion glass to greatly reduce chromatic aberrations."
An APO lens, by definition, has none AT SOME particular focusing distance.
======================


the definition of an apochromatic lens is one that corrects chromatic
aberration of 3 colors.  an achromatic lens corrects it in 2 colors.

The low dispersion glass in the 300/4.5 is the same thing Nikon
calls ED glass.

the problem with the 300/4.5 isn't the lack of low dispersion elements
but just that it is an old design telephoto.  It isn't particularly
fast at max aperture, and it doesn't focus very closely at all.

an aside-- you need an apochromatic design for faster lenses, but not
for slower, and it has to do with how acute the optical path is, so
for the shorter the focal length, the larger the aperture can be
before ED glass is needed.  at 300mm, you can get a lens well
corrected for chromatic aberration using achromatic doublets without
ED glass if the max aperture is only f/5.6.  as the max aperture gets
larger, you increasingly need ED glass to eliminated chromatic aberrations
(at the wide apertures).

j. albert

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