[Leica] Performance wide open

[Erwin Puts <imxputs@knoware.nl>]

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