[Leica] Re: Re: FS: Leitz/Leica 350mm f/4.8 Telyt-R

[henningw at archiphoto.com (Henning Wulff)]

>At 12:26 AM 2/11/2005, Frank Dernie wrote:
>
>>AFAIK there is no such thing as "APO glass". To get APO performance 
>>a combination of elements of differing characteristics and 
>>curvature is required to cancel the fact that different colours 
>>diffract differently. The unusual partial dispersion glasses which 
>>this requires in at least one element are very expensive. Whether 
>>the 350 actually has these is debatable. The fact that it is v. 
>>sharp is well documented
>>Frank
>
>
>
>Here's my reference, straight from a lens glass article about Leitz, in 
>1985:
>
>
>"Leitz manufactured APO glass at a time when other manufacturers had 
>not yet developed the calcium fluoride element."
>
>Then...
>
>"The Leitz APO Glass and the later Nikon ED glass both have similar 
>properties, most of which is a closely guarded secret. Leitz has 
>released the refractive index of the APO Glass, which, at 1.95, is 
>higher than any other glass made today. A Nikon employee once said 
>that given the chance to choose, he would choose to use the Leica 
>APO Glass for his own picture taking, which is quite an endorsement."
>
>Therefore, my reference to, and I suspect Leitz's reference to, "APO Glass."
>
>The write-up on the 350 says:
>
>"The use of special glasses from the Leitz Glass Research Laboratory 
>and the complicated arrangement of the optical components have 
>reduced the residual chromatic aberrations in the corners of the 
>picture, usual in long focal length lenses, to a level that is no 
>longer disturbing in photographic practice. The particularly good 
>detail rendering and the high contrast of the 350mm Telyt-R lens 
>also facilitates focusing in poor lighting conditions and on rapid 
>action subjects."
>
>I apologize to those that I offended by using the term "APO Glass" 
>but I was simply stating both what was said to me by Leitz and what 
>I read about Leitz. From what I read, APO Glass was invented by the 
>introduction of the calcium fluoride element. A special glass. The 
>special glass (which was called APO glass) plus the optical design, 
>produced an APO lens.
>
>JB

Well, it just shows that Leica (or Leitz) marketing doesn't know what 
it's talking about either. 'APO glass' is, as Frank has pointed out, 
nonsense. Anomalous and partial dispersion glass, or calcium flourite 
crystals, help in correcting a  lens so that its performance can be 
described as being apochromatic at certain focus distance(s), but 
that's it. It's quite possible to produce apochromatically corrected 
lenses without using such special glasses, but it's harder.

If they insist on using the term 'APO glass', consider it an 
invention which has the unfortunate characteristic of being easily 
confused with terms that have a precise and known meaning.

Almost as bad as Sigma which used the term 'APO' on lenses that 
didn't come close to that designation. But then, some Leica R lenses 
were from Sigma... :-)

-- 
    *            Henning J. Wulff
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