Hi Roy,
> 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.
Printing and capturing/manipulating the image tones are two entirely
different things. It is quite necessary, if you are going to do large tonal
manipulations, to do so to a high bit depth file, or you lose tones.
Also, that's NOT how a scanner works. The 16k potential gray levels is over
the entire density range the scanner is capable of. The scanner does not do
setpoints in hardware, that simply does not work in the scanners we use. A
scanned B&W image only occupies a SMALL portion of the 16k values. You may
only get, say, 2000 tones of valid image data with values between 1024 and
3024...and the rest of the values are not used, as there were no tones of
that value in the image. Take a look at the histogram of a raw B&W image
file, and you will clearly see this.
When you set the setpoints, these 2000 values are then mapped out to occupy
the entire range. You then apply your tonal curves to the high bit,
setpointed data.
> 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.
That is not how printing works. There is a dither/halftone pattern
involved, and the number of perceivable tones that are printed are far more
than 5. Piezo can clearly print in the hundreds, and they claim "more than
a thousand". You REALLY don't believe a quadtone printer only prints 5
perceivable levels?
Print out any gradient strip/step wedge and see that for your self. Be
aware, for B&W, your eyes are limited to between 100-200 tones for any given
light...so tones that may very well be there, you can't distinguish. But,
this is good, as it gives smooth tonality.
> 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?
Of course. It also has a different effect depending on where in the process
you are talking.
> Now if Dynamic Range were "number of levels" we'd have the situation that
> we're all spending big bucks for high dynamic range scanners and
> then on the
> way to printing, first throwing away more that 98% of the dynamic range
> in Photoshop and then throwing about another 98% of the dynamic range
> in the printer.
Yeah, but that's not what's happening...the ONLY time you need a high
DENSITY range is for slides (it's not relevant if it's high dynamic range,
simply density range, the dynamic range must only be "sufficient"), as they
have a much higher density range than negatives. Negatives have a much
smaller density range. And, as I've explained above, why you need higher
DYNAMIC range is so you can do tonal manipulations. This is more so true
for B&W than it is for color, BTW, as color is 3x8 bits, or 24 bits, and B&W
is only 8...and therefore makes more of a difference.
> That's throwing away a total of 99.97% from film to
> paper. Obviously absurd.
Yes, absurd...but as I said, that's not what actually happens.
Even if you are printing only a limited number of tones, the larger dynamic
range allows you to expand a particular tonal range you may want to have
expanded in your output. This all makes perfect sense, if you understand
it.
> Or was it that right at the end the dynamic
> range mysteriously came back?
No, the dynamic range has decreased, but, technically, you would have
maintained the overall image DENSITY range. This is a prime example of why
density range and dynamic range are not the same.
> Is dynamic range just a worthless measurement?
Perhaps to some, but not to others.
> What happened to
> the dynamic
> range during all that?
The dynamic range decreased, but that does not matter, you've used it for
what you wanted to use it for. If you only captured the data in a low
dynamic range (but with a high DENSITY range), you would not have been able
to separate out some tones you may have wanted to be separate in your print,
or manipulate the tonal range such that you don't lose/compress/combine
tones.
> The only correct answer to all the
> questions above is that dynamic range isn't defined as number of levels.
Possibly to someone who doesn't understand this, because your "answer" is
simply a wrong answer. Dynamic range IS unquestionably the number of
levels. I'll reference you to something I've already posted, but appears
you either ignored, or missed:
It CLEARLY states that the number of digitization levels is the same as
dynamic range, providing you want to digitize down to noise.
"The dynamic range is often represented as a log ratio..."
"This ratio also gives an indication of the number of digitization levels
that might be appropriate for a given sensor...Unless there is a desire to
resolve ... noise".
Obviously, if you want to resolve into noise, you increase the dynamic
range, hence the number of bits... This is just so basic, and simple, and
so clearly stated in so many places. It simply amazes me you want to argue
about it...MORE...
> Please, think about it.
Oh, I have...and I keep reaching the same conclusion ;-)
Austin
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