I think the first problem here is that "tonal scale" isn't really a precisely defined term. My understanding is that "tonal range" may be more appropriate and is sometimes used here as well. But even then, that term seems not to be well defined.
Originally Posted by Donald Miller
The 3rd edition of The Focal Encyclopedia of Photography doesn't have an entry for "Tonal Scale" or "Tonal Range." They do use the term "Tonality" and define it as, "The overall appearance of the densities of the component areas of a photograph or other image with respect to the effectiveness of the values in representing the subject."
Please correct me Donald, but I think this is what your question is about:
"Why is a higher print d-max necessary to achieve a print with greater Tonality?"
If so, then let's look at a print that has a density range of 1.5 (maybe a pt-pd print) and compare that to a print that has a density range of 2.1 (maybe an Azo print). Remembering that one stop of reflection density is equal to 0.3, our pt-pd print can reproduce a range of tones that will be compressed into a reflection density range of 5 stops, while the Azo print will reproduce a range of tones that will be compressed into a range of 7 stops.
This pt-pd print will only be able to display a "shorter" tonal range than the Azo print, which comparatively will display a "longer" tonal range. So the Azo print will have greater Tonality than the pt-pd print.
Back to our definition of tonality - "The overall appearance of the densities of the component areas of a photograph or other image with respect to the effectiveness of the values in representing the subject." Note the part about effectiveness of values in representing the subject. That means our eyes have to take part in this question.
Since our eyes can can't really get much more information out of a reflection print with a greater density range than about 2.1 or so, we can't really get a print that can display a greater range of tonality than really around 7 stops.
Using Zones as a reference - we probably can agree that there are 9 or 10 zones that can be reproduced in a print. The photographic process does this by compressing the tones. It's the toe, shoulder and overall contrast of the paper and films characterisctic curves that work to our advantage. They compress highlight and shadow densities from the original scene so that they can be reproduced in the range of tones that our printing paper can reproduce.
Even a material with a relatively low dmax like pt-pd can reproduce a wide range of tones - all 10 Zones even, but it can't do so without reproducing those tones at a reduced visual contrast. It does so by compressing even more than a paper with a greater density range. And of course, if a negative gets a minus development, it can allow the reproduction of more Zones, but again it must be at a lower overall print contrast.
Whether this meets our need for "effectiveness of the values in representing the subject", is the question to ask next.