Digital BW, The Print

From: "Austin Franklin" <darkroom@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Sunday, October 13, 2002 9:15 AM
Subject: RE: [Digital BW] Data Mapping During Scanning was: 'combed'
histograms in 16 bit ?


(snip earlier)
> >
> > Okay understood. I guess my misunderstand comes from the fact that the
> > histograms I see of raw scans, while narrow, are often considerably
larger
> > then 6% of the 16-bit range I would have expected. However since the
> > histograms are 8-bit I guess we cannot take them literally.
>
> Yeah, that is why I want to understand what on earth is going on with the
> raw data...and how it's justified.
>
> A digression:
>
> Let's say a typical film scanner has a dMax of say, 3.6...and it's got a
12
> bit A/D...  12 bits can only represent 4096 distinct values...  Now,
typical
> film scanners we use do not have adjustable front ends, they do have
> adjustable exposure time, but that doesn't change the data relativeness
that
> the sensor sees...they record the data with linear differences in density.
>
> Now, this is a tough one, and I know it doesn't make practical sense, but
> bear with me.  B&W film has a density range of say, 2...well, according to
> how our scanner works, that's only 100 discernable tones if the scanner
> records linearly throughout the entire density range, where each
discernable
> density step is the same, and that's how most all of them work as far as I
> know.

Austin,

Do we need to sort out the difference between optical and electronic dmax?
The optical dmax of most scanners is far below 3.6. I believe only the older
Howteks such as the 4500 come close at 3.5. Other scanners even the Imacon's
are more in the 2.5 to 2.8 range. So does the highest optical density a
scanner can read then get represented by the highest bit value? Does your
12-bit scanner then place its 2.7 optical dmax at 3.6?
>
> The scanner, when recording, doesn't know what the density range of the
film
> is, that it is scanning, it has to record the entire range of 3.6 for
EVERY
> film that it scans.  That assumes that it doesn't have an adjustable front
> end, and none of the typical film scanners do...  The density range of the
> film comes into place when you set the set points, you are telling the
image
> file what the density limits of the scanned film is.

That makes sense in that the optical density range of the film will most
likely be a subset within the range of the scanner's optical density
capabilities. You certainly would not want the film to have a larger range.
>
> Now back to our regularly scheduled program ;-)
>
> > So we have 2500 adjacent levels in 16-bit space. Obviously then if you
do
> > any gamma, curves or similar adjustment without first adjusting the end
> > points outward you will lose or damage the data.
>
> Exactly correct, and why you need to set setpoints first, then expand the
> setpointed image data over the larger range, to give you gaps between the
> data to allow for tonal corrections.

This is also the situation you are in when you drop from 16-bit mode to
8-bit mode in PS. All the adjacent values are filled and any adjustment
start to drop out tonal values.
>
> > pixels will be moved from
> > one level to another level and added to the pixels already at that level
> > resulting in spikes and gaps in the data reducing the number of total
> > levels.
>
> Bingo.
>
> > If you spread or remap the end points on your data set first, you create
> > room to move the pixels from one level to another without adding them to
a
> > level that already contains pixels or at least reduce the certainty to a
> > possibility.
>
> Correct.
>
> > With our example of 2500 adjecent level from 1000 to 3500 in
> > 16-bit space let's say we move the 3500 value to 65,000 and the 1000
value
> > to 10. We still only have pixels assigned to 2500 discrete levels
> > but those
> > levels are now scattered across a range of 64,000 possible levels
> > and while
> > moving them around during adjustments the chances of their
> > interferring with
> > eachother is small. Am I tracking this correctly so far?
>
> Absolutely.  That's exactly what happens.
>
(snip earlier)
>
> Hence why I like having these discussions for not only my self, but others
> to understand the theory behind this stuff.  I DO believe it helps allow
you
> to get better images.

I am sure a lot of people are groaning over our discussion at this point but
I really feel that you cannot avoid this kind of technical discussion if you
really want to grasp how these new tools work. Scanning is incredible
important to the end quality of a digital B&W print and we need to know how
the data is manipulated.

All of this makes my desire for a good file analysis tool stronger. There is
a Mac utility on the ScanHi list that count the number of discrete levels in
a 16 to 8-bit file. I wish that there was something similar for the PC.
Still want that 16-bit histogram. Even if it is as long as a bus you could
have zoom capability and be able to scroll. Even a utility that produced a
comma delimited text file of how many pixels are assigned to each level in
16-bit space would let you go in an see exactly how the data is changed from
step to step.

Thanks,
Martin

(snip)