[Leica] image evaluation criteria

[imxputs <imxputs@knoware.nl>]

My brother Hubert is a conservative and a pragmatic. He does not believe that 
progress is real and he cannot imagine that anything written since 1970 has 
any value. He also refuses to consider facts or logical reasoning that 
contradicts what he believes is true. But he is deeply involved in modern 
gadgetry (that is pragmatism). He uses these instruments as he pleases, 
whatever the intention of the designer might have been. He correctly justifies 
his approach with two arguments: it is my money and I like what I like. 
He constantly urges me to stop trying to update conventional wisdom in 
photographic lore to a level required by the actual state of the art of 
photographic and optical sciences. 
Follow the mainstream, he pleads, then your life is easy. Repeat sales reps, 
marketing brochures and every snippet of Leica lore that is floating around in 
this info-soaked world. He made these remarks after my recent visit to Solms 
and Oberkochen, where I was inundated by MTF graphs, spotdiagrams, ray fans 
and a plethora of optical aberrations that need to be corrected in a  very 
subtle and artistic way in order for us mortals to start raving  about Leica 
lenses. 
It took me three full years of testing and thousands of pictures (yes real 
pictures of real life subjects) to come to grips with the M-line of Leica 
optics. Now the daunting and very exciting task is to engage my self in the 
R-line. Another three years? Hubert tells me to use my spare time for more 
pleasant activities as studying the theoretical base of Startrek physics. 
Why am I giving my fellow luggers a peek into my family disputes? Well as 
Clinton becomes more popular the more dirty he talks and acts, I think we are 
at an intellectual and moral low point in history. Is there a chance that the 
classical Greek ideals of beauty, rational discourse and lust for truth will 
survive? At least in Startrek I presume. I am very pleased that Data has a cat 
(or the other way: the cat has Data). If ever a creature has the basic 
characteristics of a Leica M camera, it is the cat. This species survives in 
even the harshest and unfriendly environments, it never loses its character, 
it does not compromise,it is the most effective small predator in evolution 
and it is a thing of beauty. And above all: you can study it for years and not 
know anything about it. It still is a mystery, but a very nice one.
These thoughts have been inspired by the recent discussion around that most 
elusive of topics: image sharpness. This topic pops up quite often and then 
fades out without any real progress. Still the topic is of utmost importance. 
So let me try to make some observations. Sharpness does not exist in any 
objective way. It is a subjective impression of the eye/brain mechanism and 
cannot be measured or defined. Note that I in my reports never use this ord 
'sharpness'. I use the optically correct words 'contrast of fine detail' and 
'edge contrast' to describe image chracteristics. Generally we may surmise 
both words under the umbrella word "clarity".

When a designer creates an optical system, he/she has always a clear purpose 
in mind about what the lens has to accomplish imagewise. As any lens is always 
a compromise between many demands and variables, the resulting imaging 
characteristics can be related to the ideal lens: that is a lens without any 
defect, that will reproduce the object in front of the lens with 100% 
faithfullness in three dimensions. Let us make very clear that Leica designers 
NEVER assume that their lenses should or could be optimized for flat two 
dimensional test targets of whatever configuration. It is an undeniable fact 
that any real lens has only one, I repeat only one plane of accurate focus. 
That is by definition a plane of very thin depth. Thus evaluating a lens in 
the optical sense of the word is studying the characteristics of image points 
as they are projected on this plane of focus. It so happens that the MTF 
graphs are a very good analytical tool for just this: studying the point 
characteristics in the image plane. 
It is far beyond the truth to imply that a study of the flat image plane (that 
is what designers and serious testers do)disregards the three dimensionality 
of real life objects. To imply as some postings do, that a test of a flat 
object is irrelevant to practical photography, is to misunderstand the 
fundamental laws of optics.  There are many methods, some very complex, some 
very secret to make sure that the image characteristics that are defined in 
the plane of accurate focus can be extended to a three dimensional area, part 
of which is contrained by the angle of view of the lens and part of with is 
constrained by the depth of field of the aperture used. 
We have to make some distinctions: some tests that are proposed in common 
practice: photographing a newspaper page, photographing one of many testcharts 
(bar lines in many configurations) are not very meaningfull. This is not the 
fault of its inherent two dimensonality, but because the target as such is not 
very well correlated to actual imaging chararteristics of a lens. In the case 
of  standardized test patterns, we have the problem that most if not all users 
lack the necessary background to interpret what they see or think to see.
Again we should not ridicule a methodology because it is falsely used.

An expert designer can predict with a high level of accuracy how a lens will 
perform, just by looking at all his/her figures churned out by the computer. 
We should really bring ourselves to a higher level of awareness that the 
twodimensional test target versus three dimensional reality confrontation is 
not a meaningfull  distinction. We have bad tests and good tests and bad 
testers and good testers.

Now the issue of evaluation. Many postings assume that testing a lens in any 
objective way (that is using tests that produce contrast or resolution 
data)has no relevance to the demands and requirements of working 
photographers. First of all I hope to have made clear that measurement of 
image performance is MUCH more than looking at 'sharpness' (which is a 
non-issue as it is not existing). A responsable designer and his companion the 
tester will choose very carefully those measurable characteristics that are 
very relevant to the image as required by working photographers. To assume 
that testers are otherwordly and/or insensitive  of the needs of real 
photographers is doing them injustice. Again we have good and bad.....

There does not exist any evaluation method that can handle in one merit figure 
all the many characteristics of a lens.  And here indeed the acceptance of the 
working photographer is the last word. The role of the tester is to bring in 
figures based on carefully conducted tests to support or inform the user of he 
validity of his final decision. Or to help him make a meaningfull choice.

There is no us-them situation. A tester might be a very good photographer 
himherself and a working photographer can be a lousy tester. It is the role of 
a tester (at least that is the way I see and practice it)to inform a working 
photographer of the potential image qualities of a lens based on scientific 
testing of, yes, two AND three dimensional objects. If a working potographer 
likes or needs or is interested in these characteristics is purely his 
nondebatable choice.  The image qualities I try to investigate are very 
relevant to practical phtography. Again: some of my test objects are indeed 
the barline patterns of a flat plane. If you really know how to translate 
these results into the demands of photographing cats and girls and snow storms 
and emotions, then there is nothing wrong with this method.

Have a very happy new year where at last we can do what we need to do: use the 
imaging capabilities of Leica lenses to start recording the beginnings of  the 
third millennium in its myriad ways.

Erwin