[Leica] sharpness & optical quality

[Erwin Puts <imxputs@knoware.nl>]

>  In other words: sharpness is NOT everything
>  in photography. What about "residual" tonal gradation,
>  shadow detail, highlights' subtlety, color accuracy when its acutance /
>  edge resolution goes down somewhat at f/2.8 or f/2 ?

>Right on. When will people realise this? Most modern lenses are 'sharp'
>enough. The occasional massive step forward in resolution (35 lux -> 35 lux
>asph) excepted. The character resides in the rest of their qualities. Add
>bokeh to the above list, for a start. Judging a lens by its sharpness is
>like judging a car by the size of its engine.

These comments can be read and heard often. They are not 
representative of current thinking about image quality and 
demonstrate an embarrassing lack of insight in the true nature of the 
topics discussed.
I cannot think of any person or book discussing optical quality in a 
serious way who will uphold the notion that "sharpness" has relevance 
to image quality. Sharpness is often equated with resolution, which 
is not the case. No one has ever been able to define 'sharpness' in a 
consistent or measurable way. Fact is that "sharpness" does not 
exist. The notion of sharpness impression does exist, but that is a 
psychological phenomenon, loosely related to acutance. Resolution 
only refers to the ability to distinguish between two adjacent 
objects, the smaller the distance between two objects, the higher the 
resolution. It has no direct relation to image quality, but PopPhoto 
still uses it as a criterion for optical performance. The measurement 
of resolution is so dependent on so many uncontrollable parameters, 
that no one would propose resolution as a discriminating 
characteristic for optical systems. The suggestion that the Summilux 
asph has a much higher resolution value than the previous version is 
not true. Contrast is higher, but not resolution, which is only 
marginally improved due to the higher contrast.
It is remarkable that the sharpness topic is discussed often by 
persons  who  wish to denounce the value of the concept, while most 
persons who discuss optical quality do not even think of using this 
concept.
Now bokeh, which is just a new word for the older concept of 
'rendition of unsharpness areas' is a very imprecise notion and is 
based on perception and personal judgment and appreciation. The 
sharpness impression is also imprecise and based on perception etc. 
So if one argues in favour of bokeh or highlights' subtlety or 
residual tonal gradation (whatever that may be) or any other 
impressionistic criterion, we are not proposing anything new. I do 
not see the added value in replacing the vague and obsolete notion of 
sharpness with another concept just as vague and unreliably related 
to the real optical quality of a lens.
A lens cannot be characterized by one simple criterion, measurable or 
not. And the definition and assessment of the optical quality  of a 
lens is a major undertaking that defies any attempt to simplification.
Indeed a lens has a character, just like an individual, but we are 
all aware that the description of a person's character is a highly 
subjective and dangerous activity. So is the description of the 
character of a lens.
The recent discussion about the perceived differences between the 
Nokton and Summilux 50 clearly demonstrates the pitfalls. The Nokton 
lens was not allowed to show its qualities in that approach.
When proposing characteristics for evaluation of a lens, we should be 
aware that this area of discourse can be studied from several 
perspectives and levels. Any  photographer can choose whatever lens 
(s)he wants, based on whatever  arguments. A discussion of these 
choices  may be enlightening as we get a glimpse into a 
photographers' personal decision chain. This is however no substitute 
for a comprehensive assessment of the optical quality of a lens, 
based on current thinking and theory of image quality.


Erwin