[filmscanners] Number of colors (tones) was: Re: Dynamic range

["Preston Earle" <PEarle@triad.rr.com>]

"Roy Harrington" <roy@harrington.com> wrote:
"Consider the current state-of-the-art scanning and printing that we are
all involved with.  The current crop of high quality pro/consumer
scanners are all about 14-bit depth.  That gives 16384 potential gray
levels.  The demand is obviously there for this bit depth capability.
However, virtually everyone who wants the high bit scanners is perfectly
content with printing using only 8 bit files -- i.e. 256 levels.  Going
further, the printers themselves don't print 256 levels.  Some of the
best results these days are quadtone inkjets.  That's 4 levels of gray
ink plus of course the white of the paper i.e. 5 levels total.  Each dot
on the paper can be one of 5 possible gray values.  So, we have images
going from 16384 levels to 256 levels to 5 levels -- and we're all
pretty happy with the results.  So I ask you and anyone else is "number
of levels" much good at characterizing what's happened to the image on
the way from film to paper?"
------------

This isn't exactly right. There are a couple of issues:

Bit depth: It isn't so important how many bits a scanner records, but
rather how good those bits are. I suspect most consumer (i.e. under
$10,000) scanners have difficulty "seeing" 8 bits deep. They can't
reliably discern the difference in a 253 and 254 shadow value (or is
that a 2 and 3 shadow value? I never can remember.). Whether they report
the value as 253.34 or 253.35 is immaterial.

Number of tones: A 14-bit number can represent 16,384 potential gray
levels in each color. For a three-color device, that's what, 4 trillion
possible combinations? An 8-bit, 3-color device theoretically displays
16 million colors. Human vision can perceive something like 8,000
colors, so there are lots of "different colors" we can't tell apart.

Quad tone printing: I don't know directly about these printers, but I
suspect the four inks, different shades of black, are used to extend the
tonal range of the print, just as commercial printers use 4-color
"grayscale" or double-black duotones to make blacks blacker and other
tones "smoother" than the 256 tones available with plain-old-black.
Again, theoretically, there would be 16,384^^4 different grays available
("billions and billions"), but we could only see maybe a thousand of so
of them.

I don't know nothing about no stinking Dynamic Range.

Preston Earle
PEarle@triad.rr.com


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