[Leica] OT: which enlarger lens

[henningw at archiphoto.com (Henning Wulff)]

At 2:47 PM -0800 11/12/05, Mark Rabiner wrote:
>On 11/12/05 6:11 AM, "Frank Filippone" <red735i@earthlink.net> typed:
>
>>  I did not say that specifically... the 2 lenses are considered of equal
>>  quality.  Whether one is APO or not is a matter for the seamanship folk.
>>
>>  Probably your second idea is more true...
>>
>>  Frank Filippone
>>  red735i@earthlink.net
>>
>>  So really a Componon S is considered APO?
>>  Or the APO Rodagon is maybe not so much APO but just a really good lens?
>>
>>
>As to enlarger lenses not being APO the whole APO thing started from process
>camera lenses which should be a close kin to enlarger configurations. So why
>not?
>

Process lenses need to have exactly the same magnification at three 
specific wavelengths, corresponding to the 3 process colour 
separation filters used. Any differential magnification causes 
misalignment and lousy quality. The way to get this is to have the 
lens corrected so that at a given reproduction ratio these three 
wavelengths are in focus with no distortion (differential 
magnification across the image field). These are the 'Apochromatic' 
lenses that are the basis for further discussions.

After a while everyone in the photographic world 'knew' that apo 
lenses were the 'best', so marketers got in on it and made the most 
of it and confused everyone. Remember the early Sigma 'APO's? The 
rest is history.

Now just because 3 wavelengths are in focus at the same time at a 
certain magnification and there is no distortion does not mean the 
lens is good or even useful for ordinary photography. The other 
wavelengths may fall where they may, and their focal point may 
diverge wildly from that of the 3 wavelengths of interest, and as 
soon as you get out of the optimum magnification range the image may, 
and sometimes does, fall apart completely.

In the same way some of the highest resolution lenses ever made, 
namely those intended for imaging computer chips only have to handle 
one wavelength and magnification, and may therefore be totally 
useless for general photography; they may not even be achromats which 
focus 2 wavelengths at the same plane. They are often very good 
lenses for other purposes, but that is more an accidental byproduct 
of the huge effort to make them perform well at their intended task.

Lenses for general photography have to have even performance over the 
whole spectrum and for the full range of magnification that the lens 
is to be used for, have to have reasonably large apertures, be not 
too big or expensive, etc. Leica lenses have generally adhered to 
this principle, and that is why they work well, but cost more than 
average. The new 'APO' lenses that Leica makes are likely not 
apochromatic over their full magnification range, but likely are for 
at least part because Leica is not known to bandy unmerited terms 
about as readily as some other companies, but the most important 
thing is that the rest of the visible wavelengths are focussed very 
close to the same plane, and all the other aberrations are handled 
well. The 'APO' designation itself is both misleading and irrelevant 
for general photography.

The 'APO' designation of the first APO Rodagon certainly was such a 
designation, and had all to do with marketing. It was a good lens, 
but not as good as the Focatar from 1958 I had, which was not 
designated 'APO'.


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    *            Henning J. Wulff
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