Re: physiology vs. sensitometry

Beakman (beakman@netcom.com)
Thu, 6 Jun 1996 12:08:54 -0700 (PDT)

> 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.

I think that the apparent visual difference is primarily a result of
different "compression-expansion" ratios of platinum and silver.

Suppose we have an outdoor scene with a brightness range of 10 stops. We
know that we can record 10 stops of information on our film.

However, if we are printing on a silver-based paper we know that the
paper can handle maybe 5 stops at best. Therefore, we either process our
film to increase contrast, or we use a high-contrast paper. Either way,
what we have done is "squash" the ten stops of information into the 5
stop range of the paper.

On the other hand, all ten stops may be printed on platinum paper as
distinct tones.

So suppose you have something in zone 5 and something next to it in zone
6. These two tones would print as seperate tones on the platinum paper,
but but might only print as one tone on the silver paper. In other
words, squashing the 10 stop range into the 5 stop latitude of the silver
paper results in tones 1&2 printing as one tone, tones 3&4 printing as
the next tone, etc. This is why we can get such good seperation in the
shadows and highlights in platinum, whereas the same print in silver just
seems crude and indelicate.

Therefore, it doesn't matter what the final Dmax of the paper is, what
really matters is what range of negative densitiy can the paper accept.

I hope this message is understandable and coherent, I'm running a bit of
a fever today so I'm a bit fuzzy in the head.

David