[Leica] Performance wide open

[Gary Elshaw <gary.elshaw@vuw.ac.nz>]

Thank you for this contribution, Erwin. Since reading postings such 
as this one i've been fascinated and curious to find out what 
properties can be corrected in the designs of certain lenses by 
changing the size or scale of a lens' design. For example, the 
Noctilux is a very large lens, and from reading your posts about its 
construction, its size is a determining factor for helping correct 
aberrations and optimizing its performance.

I'm sure i'm being overly simplistic with my thinking, but I guess my 
question is this:

If an increase in size to, for example's sake, a Summilux was made, 
could there be less aberrations in the lens and a greater optimal 
performance? I realise the consequences of carrying a Telyt-sized 
Summilux is not for everyone, but i'm curious.

Thanks again Erwin, for all of your work!
Take care,
Gary




Erwin eloquently wrote:

It has been noted on the Lug:
>With all due respect, no lens performs better wide open than stopped down.
>While some may perform better than others wide open, none will ever be
>better wide open than at say F8.

This is a remark that might have relevance in the very distant past.
Today it is not true as a general statement and it definitely is not
true for Leica lenses.
As examples: the Apo-Summicron-R 2/180, the Apo-Elmarit 2.8/180 and
the Apo- Telyt-R 4/280 actually are at their best wide open and loose
some quality when stopping down.
The Apo-Telyt-M 3.4/135 slightly improves when stopping down to 4 and
that is it. The Elmarit-M 2.8/24 is at its optimum at 4 and drops in
quality when stopped down to 5.6.  So is the Apo-Summicron-M 2/90
asph.
I could go on and on.
It is also not correct that some Leica are specifically optimized for
best performance at full aperture.  Such a statement could imply that
Leica designers have a choice in what aperture to optimize for. The
general rule is that they aim for best performance at the widest
possible aperture. But the inherent nature of optical  designs means
that there is quite often some aberration content that could not or
cannot be corrected. It such a case the designer by nature is forced
to accept an image degradation at full aperture. Stopping down
eliminates some of the rays from the edges of the field and so
automatically lowers the aberration content. Older lenses indeed
improve on stopping down to 8 or 11. When stopped down to this small
hole in the wall, the paraxial optics takes over and now we have
always good image quality. But the good quality is not the result of
a conscious design but of geometrical optical properties. Any design
program assumes you define the maximum aperture of the system and
from then on you correct the system for this aperture. Then you see
what happens when stopping down. Based on these results you optimize.
There are some older lenses (the older 1.4/50 designs as example that
hardly improve on stopping down). The illusion of improved image
quality is just the effect of the depth of field mechanism.

Erwin
"The difficulty now is that unexceptional adults believe the loss of 
youthful dreaming is itself "growing up," as though adulthood were 
the passive conclusion to a doomed activity and hope during 
adolescence."

 
OO             The Uses of Disorder
[_]<|          Personal Identity and City Life -- Richard Sennett
  /|\
Gary Elshaw
Post-Grad Film Student
Victoria University
New Zealand
http://elshaw.tripod.com/