>========================
>> 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
>
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.
Winsor
Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
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