[Leica] Sharpness and tonality

["Erwin Puts" <imxputs@knoware.nl>]

It has been proposed on this list that 'tonality' is more important than
'sharpness'. But do we really know what we are talking about. These are
fuzzy terms of which we assume the user knows what he is talking about. On
the assumption that these concepts are related to the finished print (one of
the endresults of the photographic imaging chain), let us delve somewhat
deeper into these terms. Are they meaningful and are they indeed at
different ends of a scale. 'Sharpness' as most will know does not exist as
such. We have a sharpness impression as our visual system scans a pattern
and the sharpness impression is greatest if there are steep bounderies
between adjacent areas of equal brightness. We can measure these edges with
a microdensitometer and it is commonly called acutance, correctly describing
the phenomenon.Websters Dictionary defines acutance as a measure of the
steepness or abruptness of an edge in a photographic image. At least Webster
knows what photographers are talking about.
Now 'tonality'. Any print has a maximum white and maximum black, that can be
measured: whiter than the physical base of the whitening agent in the
emulsion is impossible and blacker than the full expose of all silver is
impossible too. So the maximum tonal range lays always between pure white
and black. To stay within the realm of measurements first, it is clear than
the progression from white to black  happens in steps of more or less grey.
Fechner's Law states that the eye can only discriminate between two values
of grey if the relative difference is 2%. Now evading the equations as they
involve logarithms, it can be calculated that between the pure white and
black there are 243 different grey tones (values) that an observer can
distinguish in a print. We can define tonality as the maximum range of grey
tones that can be distinguished and a good print should be such that all
these shades of grey are present. If a print has a lesser range of grey
shades, we may say that such a print has a limited tonality.
Comparing both concepts acutance or edge steepness (sharpness) and maximum
range of grey values (tonality), is there anything that suggests that both
are at odds with each other. Not at all! Both are present in a print and it
may even be argued that both do enhance each other.
The implication behind the statement might be  that some lenses are better
in the rendering of tonality than in definition of sharpness. This would be
wrong!A lens than can record fine details with high clarity will also record
any subtle differences in gery value, where a lesser lens would have flare
and low microcontrast and other defects than would wash out fine differences
in tonal values.
So a lens good for the recording of fine details (acutance) is also good for
tonality.
Back to the original proposition: sharpness and tonality can not be used as
two concepts that are mutually exclusive and not even as a slider on a
scale: more of A means less of B.
It is the fuzziness in the original statement that may produce confusion in
sketching a dichotomy that does exist.

Erwin