Re: 16 bits = 15 bits in Photoshop?
Re: 16 bits = 15 bits in Photoshop?
- Subject: Re: 16 bits = 15 bits in Photoshop?
- From: bruce fraser <email@hidden>
- Date: Sat, 16 Apr 2005 09:55:58 -0700
At 8:14 AM -0400 4/16/05, email@hidden wrote:
<snip>
That is *not* a comparison of 8-bit vs. 16-bit editing. You have Image A in
Camera Raw. You export it as Image B, unedited, to Photoshop, where
you convert
it to 8-bit. You then return to Image A and edit in in Camera Raw, thus
applying a change that cannot be duplicated in Image B. You then
turn to Image B
and edit it in Photoshop, thus applying a change that cannot be duplicated in
Image A. Having ensured that the test is utterly meaningless because the
competing files are no longer comparable, you then export Image A to
Photoshop and
produce a scintillating demonstration that would, (if the images you
showed were
larger than 2x2) prove beyond all doubt that applying the same curve to two
different files produces two different results.
The only difference between the two is that on one, I edited the
high-bit data in Camera Raw, and continued to edit the high-bit data
in Photoshop, where in the other, I didn't do any editing on the
high-bit data. Both files had the same demosaicing, the same color
space conversion, and the same white balance. ( if I hadn't applied
the same white balance in Camera Raw, the advantage of the high-bit
file would have been even greater).
I already said, in 2001, (thanks for reminding me) that it doesn't
matter what you use to edit the high-bit data. That was in the
context of scanner drivers, but Camera Raw is, in this context, just
another opportunity to edit the high-bit data. All I did in Camera
Raw was some rough tonal shaping, not even big moves, and the low-
and high-bit versions of the image on p22 look extremely similar, to
me at least. Yet just one identical curve application to these
seemingly-identical images makes them look very different, even at
2x2.
If you want a valid comparison, just export Image A from Camera Raw when
you're ready. Then make a copy of it as Image B and convert to 8-bit. At that
point you're good to go. You have two equal images. Do your damndest to break
Image B--but whatever you do, you have to do to the 16-bit Image A
as well. When
you're finished, convert Image A to 8-bit, and then you'll have a "night and
day" difference when you compare A and B--or so the guy who is using your name
says.
No, that's just a silly game. A much better test would be to get an
optimally-edited high-bit image from a much better photographer than
I am, then take an 8-bit version of the original uncorrected image
and try to edit it to produce a result that she finds equally
satisfying. (You can even start with a gamma-encoded version rather
than going all the way back to the linear data.) If you do that test,
I'll sit up and take notice.
Showing people two versions of an image, when they don't know what
it's supposed to look like anyway, and seeing how many pick the
high-bit over the low-bit, isn't really testing anything.
Now that I know that you aren't the one who said "it's totally obvious to
anyone who looks that it's very advantageous to do the big moves on high-bit
data. " I guess the point will have to be conceded.
No, I said that (four years ago). I guess I should have added the
important qualifier that the person doing the looking has to know
what they're looking for. The remarks were directed at people working
on their own images. It remains obvious to me that I can get more out
of my images when I work on them in high-bit form than when I don't.
On some images, the difference is indeed night and day.
As for what the eye sees, of course I agree. For example, you or a person
using your name once wrote, "For 8-bit-per-channel images, I always
use a Curves
Adjustment layer...because when you use Adjustment Layers to edit the image,
all the edits are calculated at the same time when you flatten the image. This
approach degrades the image much less than burning successive rounds of
corrections into the image one by one. . . ."
I wrote that many years ago, and I was wrong, the victim either of
bad information or wishful thinking, I forget which. The adjustment
layers are applied sequentially.
The argument has never been that the same edits applied to 8-bit and
16-bit will produce better results on the 16-bit version (though they
sometimes may). The argument is that if you know what you're looking
for, you'll get it more easily out of the 16-bit than the 8-bit
version, typically with fewer edits, AND you can use things like
graduated blend mode adjustments applied either through masked layers
or through the History brush set to a blend mode, that work much less
well in 8-bit mode.
However, since I like to check my sanity every now and again, last
night I went through an exercise where I took an unadjusted scan and
edited it in high-bit. Then I applied the exact same edits to an
8-bit version. The 8-bit version shows large hue shifts that are
absent in the high-bit version. I'm happy to send you JPEGs of the
final results, just let me know where.
--
email@hidden
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