[Leica] lens evaluation revisited.

[Erwin Puts <imxputs@knoware.nl>]

Any object in front of the lens (or the eye) is just a random pattern of
patches of varying brightness (and colour), shapes and extensions. That is
what reaches the retina or the emulsion. The prime directive of the lens
designer is to ensure that this random pattern is recorded as faithfully as
possible. No more,no less.
BTW: this pattern is also the basis for exposure metering and much of the
discussion on relective/incident metering would benefit from this
perspective.
For the eye this pattern is the starting point. The pattern recogntion
mechanism of the mind will interprete the random patches as a cat, or a
baywatch girl or the interior of the Louvre. The next step is another
cognitive one: we attach emotions to what we interpret.We dislike the girl,
we like the cat. This is part of our cultural training and our sense of
symbolism.
This cultural interpretation is subject to a vast literature of scholarly
works (Eric mentioned just one). This has basically nothing to do with the
designers prime directive.
In the course of history the lens designers have tried to fullfill this
directive more or less succesfully and in different ways. But bottom line
no designer would use a different optical formula or even a different
paradigm.
Fact is that the measure of the degree of faithfullness can be objectively
ascertained. This, again, has nothing to do with cultural influence or
personal opinion. You can like odr dislike the way this faithfull recording
has been accomplished. Witness the discussion between admirers of the
Sonnar way or the Summar way.
This discussion then is limited to the measurable part of how close the
prime directive has been fullfilled. Some choices are nesessary (resoultion
plane or contrast plane (see my companion post). But in essence no emotions
are involved. The perfect lens not being invented there is a certain
bandwidth of choices and balances. (Leica versus Canon versus Zeiss).
This approach to lens design is valuable and objective. It has not yet any
relation to the way a picture can be interpreted culturally.
Now the prime directive of a photographer is to create pictures with
meaning and purpose within the cultural context the pictures are likely to
be viewed. Second part of the directive is to develop a visual language and
vocabulary in order to express oneself more eloquently. Here we are in the
relam of language and symbolism. Ever heard Cartier-Bresson saying
something about the quality of Leica lenses?
Of course a photographer, in following the directive, can opt for a lens
ssystem with certain optical characteristics, but still we can clarly
distinguish between the optical and expressive part.
Now it is a matter of debate if lenses with certain optical characteristics
may add or distract from the clarity of the visual statement a photographer
is trying to make. This I presume is what Alf is referring to when he
speaks of certain lenses as being better suited for his way of photography.
I do not feel qualified to add anything substantial to this kind of
reasoning. I do however see the need for this dicussion.
I feel more at home discussing the optical designer's prime directive
without  the additional topics of of visual language or interpretations of
these statements.
The discussion can be complicated, but will be purely ad random unless we
learn to separate the several equally interesting topics.
On MTF graphs a I will say later on more. Time is up
Erwin