Mark,
I think I have an idea for a short and simple test
one can do in PS and possibly other software as well.
Say we start with a hi bit color image ie. anything
above 8 bit and lets call it image A (an 8 bit original would also work). Make a
copy of it and convert it to 8 bit and lets call it image B. Next do exactly the
same editing on both image as long as image A remains in 16 (15) bit, when your done you save both image A and B
and then convert image A to 8 bit mode. Then use Apply Image>Subtract>Offset 128 to compare
image A to image B and then check the mean and standard deviation of the
resulting image. The mean should be near 128 and the standard
deviation will give you a measure for the test. Values of 128
(mean) and 0 (stdDev) mean a perfect match which you probably wont get but
now we need to interpret the actual difference in terms of quality which may not
be so easy in RGB mode but in Lab mode one could do the following, reload your
saved image A and B, convert A to 8 bit and then convert each to Lab. then Apply Image>Subtract with no offset this time, if the
L* mean is zero, try again but this time instead of applying image A to image B
do the opposite (B to A) then record the mean and stdDev of each
channel, assuming it is possible then you would have:
L* (L*mean, L*stdDev)
a* (a*mean, a*stdDev)
b* (b*mean, b*stdDev)
now do the following:
(((L*mean)/ L*stdDev)^2 +
((a*mean)/ a*stdDev)^2 +
((b*mean)/ b*stdDev)^2)^(1/2)
Now we can now interpret the resulting value (~=
CIE delta Error 1976),
if the value is smaller or equal to 1, this
means the difference is imperceptible in other words the human eye can't see the
difference
if the value is above 1 but below say 4 then we
could say the difference is negligable
if the value is above 4 then we could say the
difference significant
Since I divided by the standard deviation above the
resulting value is not exactly the CIE dE1976 (square root(L*^2 + a*^2 +
b*^2)) and then the interpretation I gave is not technically correct.
We would need to use a more elaborate statistical method because we are dealing
with means and std dev and not with simple numbers as the CIE dE implies.
But if the standard deviations are small, (less then 1) then this will increase
the overall result and not reduce it which make the above interpretation even
more valid. If the standard deviations are larger then 1 then we could say right
away that the difference between our image is probably significant.
We could use this method to test B&W image as
well, when editing is done convert both image to RGB then to Lab or
directly to Lab if possible and proceed as above to evaluate.
Ideally, this would need to be done on more then
one image, 30 or more would give us what is called a confidence interval that
the difference is or is not significant based on the (variant) CIE dE method I
used. Then some could say this method (CIE dE1976) may not be so good to begin
with and I couldn't contest that but I could say this method is simple to
use because the math is relatively simple and especially if the CIE dE
value is small then more elaborate dE calculations wouldn't significantly
improve on this simple test result.
If I'm aloud an "educated" guess, I wouldn't be
surprised if there is no significant difference between editing in 16 bit mode
compared with editing in 8 bit mode and I also wouldn't be surprised to ear
that most proponant of 16 bit work would say I'll stick with my current
workflow. In fact, I'll probably do the same because however small the
difference may be it could make the difference between a fine print and an
ordinary one...
Happy holidays to all
Yves
 Original Message 
Sent: Sunday, December 16, 2007 7:01
PM
Subject: Re: OT: 16 bit editing myth or
reality?
Hi Yves,
I think your inquiry is
interesting—especially the cumulative effect that adjustment layers would have
on file data.
In the mean time, I have no doubt that 16 bit is far
superior—and even to take it a "bit" further, I would venture to say that 16
bit RGB is better than 16 bit grayscale when working on the same "grayscale"
file—the numbers seem to work better. Bruce Fraser confirmed that with
me (before he passed away). I had contacted him when I was writing my
book about some anomalies I found working with 16 bit gray scale
files.
Anyway, good luck with your inquiry, I'll look forward to
hearing your conclusions—in the mean time I have no doubt that working with 16
bit capture files is far superior to 8 bit.
Try doing some sort of
series that makes an adjustment followed by one that reverses it and see how
much error accumulates. BTW, it has been suggested that even rotating a
file can lead to some sort of degradation of data.
Best Wishes, Mark
Nelson
Precision Digital Negatives 
The System PDNPrint
Forum at Yahoo Groups www.MarkINelsonPhoto.com In a message dated
12/16/07 4:31:46 PM, gauvreauyves@cgocable.ca writes:
Mark,
You could be right but I (we)
don't know if PS works with real number until the data needs to be save or
whatever. If it's not the case and this would be surprising, then each
editing step would effect the data and each additional editing steps
would have a cumulative effect. I don't think what you propose below is
correct even if PS works with real number because the starting point would
be different.
Let's say I choose arbitraly a
normalised [0..1] pixel level of 0.5, lets see what our
starting value could be in 8 bit resolution. This would mean we started
with a value of 0.5*255=127.5 => 127 or 128, now in 16 bit this would
be 0.5*65535=32767.5 => 32767 or 32768. Lets say our original pixel
value is 32767, PS would transform this in 32767/65535 = 0.49999237 and we
could proceed with editing from this value but if we convert this value to 8
bit and save it for later comparison, PS would need to convert this to a
value to an integer value between [0..255], so lets do this by
0.49999237 * 255 = 127.4980545 and to 127 by rounding. If we need to
apply the same editing to the 8 bit data file, PS will convert 127 to
127/255 = 0.498039215, a difference of 0.001953155, though very small
this difference could be increased by editing the image further. Just out of
curiosity say we apply a small gamma correction of say 1/1.45 and see
what effect it as.
0.49999237^(1/1.45) =
0.6199955
0.4980392^(1/1.45) = 0.6183242
for a difference of
0.001671312
The difference is now lower, this
could be surprising and it's why a full analysis including both color errors
and quantization errors migth produce interesting results. I made a search
on Google to see if someone did this kind of analysis but as of now (4) I
saw only perceptual comparisons and no number crunching but interestingly
the few test I've seen seem to indicate the difference are
imperceptible from 8 or 16 bit editing on printed image. Food for thought
don't you think?
Regards
Yves
PS I don't recall any
discussion of this topic in particular but I have a bad memory so don't take
my word for it.
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