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[filmscanners] Re: Density vs Dynamic range



on 6/11/02 3:34 PM, Austin Franklin wrote:

> I certainly don't mind continuing explaining anything I can to someone who
> is genuinely interested in understanding/discussing the issue (ask Todd ;-).

Yes, now I understand the topic well enough to try to steer clear of it. ;-)

No, I think the concept makes a lot of sense for uniform and predictable
systems. I just think the application of the concept is unwieldy for many
devices/objects. IOW, it's pretty simple when the noise is stable and
uniform across the useable range, but it can be hell if it's not.

For instance, noise in this discussion has been defined as the consistency
of voltage off the CCD, which would only show itself as spurious pixel
values in the dense regions of the film, right? Sort of, something like
that? But in a scanner like my Leaf, in reality, "noise" (which really seems
to be WHATEVER may limit subtle differentiation between tones) may vary
wildly across the range. For instance, in addition to the aforementioned
voltage inconsistencies, in the least dense areas of the film the scanner's
"noise" might really be from CCD bloom; in midtones it could be red channel
smear; chromatic aberration from the lens could show up in various places,
transport problems....etc. Thus, it would be a real bitch to characterize
and integrate all that noise variation.

So I felt that (min - max)/noise was just too simplistic to really tell us
much in the way of a true "tone count" within the range. My argument was
less against the formula and the concept, than the extension of the formula
to yield a meaningful tone count. The formula insinuates that every possible
tone within the range would be present, when I suggest it may not be. Take a
scan of lithograph film. High density range, low noise, but still a low tone
count, and the tones count is not limited by the noise. So what do we have?
A low dynamic range original, scanned on a high dynamic range device,
yielding a low dynamic range digital file. I believe if you were to apply
your DyR (my abbreviation for dynamic range) formula to the file (big max -
little min)/low noise, you'd think the file had a wide range of tones, which
it does not. Therefor I feel for a tone count other methods, like pixel
counts and histograms, are more useful.

However, that there is a CONCEPT of a range which can be divided into steps,
and that Dynamic Range is a ratio of the range over the step size, is a
worthwhile notion to understand. The rest is just defining what constitutes
max, min, and noise in each application or device. (Though, I still think
there is a theoretical argument against step sizes at all (short of quantum
physics ;-) in the analog realm...)

Anyway, I must say you really are a trooper for sticking with these
discussions as long as you do! Most people would posit their concept and
walk away, not caring if the rest of the world "gets it" or not. You may be
a brainiac, but you're a brainiac with heart!!!

Todd
(wanna be geek who sucks at math turned artsy fartsy nobody who always seems
to find himself on the wrong side of arcane technical discussions)

PS, That'll teach you to invoke my name...

PPS, Looking forward to that write-up. ;-)

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