[Leica] some revealing remarks

[Erwin Puts <imxputs@knoware.nl>]

The current discussion about the relative merits of the 3.4/180 lens 
shows that the Leica myth is still alive. The notion that this lens 
gives best performance at infinity setting is not unique for this 
lens. Most lenses (Leica and others) perform best at infinity as this 
is the default setting for optical corrections. The performance of 
the 3.4/180 is quite good in itself, but not outstandingly  so, 
relative to the current APO 2.8/180. After twenty years of tinkering 
with the residual or secondary spectrum, Leica designers have learned 
a trick of two. Non-APO telephoto-lenses are of relatively low 
contrast and that hinders the definition of fine detail. The low 
contrast is caused by the lateral and longitudinal chromatic errors 
that become larger and larger when the focal length is increased. So 
an apochromatic correction may enhance contrast significantly. A rule 
of thumb notes that for a non-apo lens the longitudinal difference 
between the blue and red part of the spectrum is focal length/1000. 
That is the distance between the location of the red and blue rays 
on/along the optical axis is 0.18mm ( for the 180mm lens). Apo 
correction can reduce this to f/4500, that is 0.04mm. Now the 
classical rule for the diameter of the disc of confusion is 0.03mm on 
film. The Apo-Telyt-R 3.4/180 then delivers a level of definition of 
detail that is comparable to a very good 2/50mm lens. At almost 4 
times enlargement relative to the 50mm the fineness of detail is 
enhanced of course.
Looking at the specs from Leica itself they note (in 1976) that the 
good performance of the Apo-Telyt 3.4/180 is ONLY superior to the 
companion 2.8/180 (non-apo) at 3.4. Leica literature notes that at 
f/4 both lenses have equal performance. Now that is quite strange. At 
f/4 the 2.8/180 is as good as the 3.4 at f/4. Now whatever Leica may 
say any lens that drops in image quality after just a half stop is a 
queer animal. The current APO-Telyt-M 3.4/135 certainly has a much 
different behavior. If the performance does not drop but stays the 
same, then the 2.8/180 is at least as good as the 3.4/180 after one 
stop.
Is the 3.4/180 that good or is the old 2.8/180 better than 
suggested?. If a non-apo lens stopped down one stop gets the same 
quality as the highly evolved apo lens stopped down half a stop, we 
may wonder what is going on within the lens.
One solution might be the extended correction into the IR region of 
the AT 3.4/180. This makes it indeed a candidate for long distance 
photography as there the IR region is important and for surveillance 
areas where IR also is of importance. The red part of the spectrum is 
significant in tungsten light. So  it might be the case that this 
lens is specifically designed for a very small bandwidth of the 
spectrum, performing very well in a niche and being as a good as 
others outside the niche. Kodak Recording Film 2475 has also extended 
IR sensitivity as is positioned too in the surveillance corner. So by 
restricting the deployment of the lens to these areas where it indeed 
shines, is a wise move for Leica. But you should really appreciate 
that it is literally a specialist lens.
The Apo-Telyt-R 3.4/180 is in my view a bit overrated as a general 
purpose lens and not as good by a clear margin as the Apo-Telyt-M 
3.4/135 and the APO-Elmarit-R 2,8/180. I tested the Apo-Telyt-R 
3.4/180 some years ago and wasa bit disappointed at its performance: 
indeed a high contrast image at full aperture at a far distance but 
less impressive in normal shooting scenes than might be expected 
given the strong praise in the past.
 
Erwin