Re: physiology vs. sensitometry

Pollmeier Klaus (100561.2417@CompuServe.COM)
06 Jun 96 05:26:56 EDT

Terry, I agree that the human eye can perceive a much wider range of tones while
scanning and adapting to the details of the original scene than can be recorded
on film or even be printed on paper. We should ask an optician to know how much
it really is...
And yes: a velvet-like platinum black may look deeper than the glossy bromide
black. But no: the rain-image with glittering lights I mentioned could not have
been printed the same way on bromide paper: On the bromide, I am sure, the
viewer would have searched the 'lost' detail in the shadows, because the
material is known to be 'perfect'. I have in front of me an original Sudek
silver bromide print which I just got for restoration: Some kind of
impressionist, misty, soft landscape, sunset at a lake against the light with
some silhouettes of trees (undated). The first third of tones from white to
bright mid-grey in the sky and water and no detail in the black trees at all. I
am sure this print would be much more impressive, if Sudek had printed it in
platinum or gum. Now the shadows simply look dead and lifeless. And I am not
sure wether Irving Penn's platinums would look 'flat' when printed on silver
bromide paper. In fact similar prints - one platinum, one silver - were hanging
side by side in an exhibition I saw some years ago and they were all
'brilliant'. But the platinums somehow were on another level of perception...

Peter Marshal wrote: "The number of tones is not really a measurable entity in
any case, but it is one of the choices we may make - if you like by the zone
system - on both silver and platinum prints.)
There are various tools we can use - such as step wedges. I think it is
important to realise what they do tell us and also their limitations."

Numbers of tones are measurable: Although one could say that there are as many
tones between logD 1.3 and 1.6 as there are between 0.1 and 2.0 (i. e.
(theoretically an indefinite number) physiologically it's not indefinite. We
need a certain difference of density to distinguish one tone from another. 256
greysteps in the digital world are quite 'good' and a greyscale with 256 instead
of 21 steps would be regarded as very close to continous tone. So a process that
can print to a Dmax of 1.4 definitely cannot show the same number of perceivable
tones as one that prints to 2.0. If, for example, I say that for the 2.0 paper
0.008 logD is the smallest discernible difference for a single tone (2.0/255),
then a single tone in the 1.4 paper has only 0.005 logD difference and the
discernible number of tones is reduced to s. th. around 160.

Maybe that is an explanation of what makes the difference in the perception of
'perfect' bromide technology and 'imperfect' hand-coated materials: The better
the potential imaging quality (in a sensitometric sense), the less the viewer
accepts 'lost' tones and the more the photographer is forced to fullfill the
viewers requirements of seeing as much as possible from the original scene. The
'worse' the imaging quality and the less detail and tonal values are offered,
the more the viewer's phantasy is asked for, allowing to appropriate the image
to himself according to his individual experiences and imagination. That is what
both, Demachy and Kuehn had in common, although their photography was different.

As I don't have my books around here in Dessau, I can't give the exact
explanation of the Weber-Fechner law. It says that to measure the
physiologically perceived difference between certain light or sound impressions,
you cannot compare just the amount of lux or db but have to use their
logarithms. Kuhn got the honorary doctorship from the Insbruck university
because he showed that a reversed s-shape curve in photographic materials would
apply better to human perception: Although we see reality according to the
Weber-Fechner law, we want to 'remember' it in (printed or painted) images with
contrastier shadows, brighter mid-tones and contrastier highlights.

Klaus Pollmeier