[Leica] LHSA convention cluster bombs Part One The Beginning.

[henningw at archiphoto.com (Henning Wulff)]

At 8:18 PM -0800 11/4/04, Mark Rabiner wrote:
>
>>  Due to the wavelength dependancy of refraction lenses do not focus all
>>  colours in the same plane at once. Acromats are corrected such that two
>>  colours are in focus in the same plane at once, apocromats are
>>  corrected such that 3 colours are in focus at the same plane at once.
>>  Other wavelengths are not in focus causing colour fringing and lack of
>>  sharpness. The extent to which the other colours are OOF is lens design
>>  dependant. Apo lenses should be sharper than non apo.
>>  Frank
>>
>
>
>And super acromats like on the Hasselblad?
>What does THAT mean?
>
>Must be all three wavelengths of light in line with FM Radio.
>

More colours fully corrected. But, as has been mentioned before, the 
real issue is the extent of the deviance of remaining colours from 
the focal plane.

In the graphics industry, which first had the most desperate need for 
APO correction in producing colour separations that registered 
properly, only the three separation colours had to be considered, and 
the rest of the spectrum could be as far off the focal plane as it 
wanted, because it didn't come into play. Actually it did, as the 
colour filters weren't that sharp in their cutoff characteristics. It 
just didn't matter as much as in general photography. That's why some 
old APO lenses developed for the graphic arts industry weren't much 
use for general photography, even close-up photography.

It should also be noted that an apochromatic lens that is corrected 
at one imaging distance is probably not apochromatic at other 
distances. Achromats are usually achromats at all usual distances, 
but even that isn't always the case. Even superachromats might be 
reduced to achromats at very close distances. Zooms are almost 
impossible to fully correct to apochromatic standards, and I believe 
that the excellent 70-180/2.8 'APO' R lens only achieves this 
designation in a quite narrow focal length range at a reduced range 
of distances. It's still a remarkable achievement, only not really 
apochromatic under most conditions.

A lot of very high quality lenses from the major optical 
manufacturers (including Leica) weren't and weren't labelled APO 
because the other colours focussed _very_ close to the focal plane 
and longitudinal chromatic aberration really wasn't an issue. Then 
APO started to be seen as a marketing tool, and some companies, 
notably Sigma, tacked APO onto many lenses that were far from 
apochromatic. Zeiss and Leica have been fairly stringent in their use 
of the term 'APO', as has Nikon. Nikon has many outstanding lenses, 
but rightly never saw apochromatic correction as being that 
significant a feature of a photographic taking lens. In their 'ED' 
telephoto lenses, the ED signifies the use of glass which helps in a 
significant measure towards lowering chromatic aberration. That's 
what's important.

-- 
    *            Henning J. Wulff
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