[Leica] [leica] lens comparisons

[Erwin Puts <imxputs@knoware.nl>]

arative performance of Leica lenses is 
arguably the most popular theme on the Lug and elsewhere in our 
universe. The question is mostly posed in general terms: how does 
lens A compare to lens B.
The character of any Leica lens however defies such an easy inquiry. 
First of all: a Leica lens has a personality of characteristics and 
an optical fingerprint that defines its imagery that is not very easy 
to describe.
Secondly: in optical science it is not customary and in fact is 
considered as wrong to try to compare lenses with different focal 
lengths. The characteristics, the parameters and the residual 
aberration content are so different that a comparison between a 90 
and a 135mm lens is at best inconclusive and at worst meaningless.
When one wishes to restrict the comparison to lenses with the same 
focal length, we should first consider the many aspects that go into 
a meaningful analysis of lens performance.
Full aperture: overall contrast, flare, veiling glare and ghost 
images, the rendition of fine detail on axis and in the field and in 
several zones up to the extreme corners, the capabilities of this 
lens at this aperture for the rendition of finer levels of detail 
rendition. What is the finest possible detail that this lens can 
capture with a certain level of contrast.  Level of vignetting and 
sharpness/unsharpness gradient.
The performance of  the lens when stopping down.  How fast does the 
image quality improve and to what extent and where in the image field 
(axis and/or field and/or corners). When is the optimum performance 
reached, how does the image degrade when stopping because of 
diffraction after f/8.
Close-up performance at all apertures is an additional topic. As is 
color reproduction. As is shadow and highlight separation of details 
etc.
This list is not exhaustive, just indicative.
Given this amount of information that has to analyzed numerically in 
strictly controlled conditions and should be evaluated in 
user-related terminology, any simple conclusion about the performance 
of a lens does not justify the efforts of the lens designers and will 
not be helpful for anyone who wishes to make a considered buying 
decision.
Visit my Leica FAQ for more info about these topics.

Erwin