16 bit vs 8 bit (one more time, sorry)

Discussion in 'Photoshop Tutorials' started by Bill Hilton, Oct 27, 2005.

  1. Bill Hilton

    Bill Hilton Guest

    Someone posted this link on another NG from Deke McClelland ("Photoshop
    Bible" fame) which shows an image edited with only two operations
    (Levels and Auto Color) in both 8 and 16 bit, with a very noticeable
    difference. I know the lack of concrete examples is Mike's main
    complaint about 16 bit advocates so I thought I pass this along ...

    http://www.graphics.com/modules.phpname=Sections&op=viewarticle&artid=279

    Just fyi as I think minds are already made up on both sides of the
    argument, and admittedly his first move in Levels is pretty radical ...

    Bill
     
    Bill Hilton, Oct 27, 2005
    #1
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  2. E.g. with an underexposed digicamera shot you do need pretty radical
    Levels adjustment. With scanned slide film you need pretty radical
    Curves adjustment. With scanned negative you need horrible Curves and
    Levels adjustments.

    In addition, nearly all editing operations do add quantization noise
    (half of the least significant bit). This noise does accumulate (not
    directly additively, but it does accumulate) so after many editing
    operations there will be plenty of quantization noise in the image,
    easily up to two or even three lowermost bits.

    In the 16-bit/c space (that in Photoshop only has 15 bits) this
    quantization noise does not matter much, 3 lowermost bits full of
    quantization noise still leave 12 bits for the image data.

    In the 8-bit/c the 3 lowermost bits full of quantization noise leave
    only 5 bits for the image data.

    Btw , in photographic workflow one does not need CS or CS2 for 16bit/c
    editing, I still use version 7 and fully 16-bit/c work is very easy
    with it.

    Timo Autiokari http://www.aim-dtp.net/
     
    Timo Autiokari, Oct 27, 2005
    #2
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  3. Bill Hilton

    toby Guest

    There is no debate here. Photoshop provides 8 bit and 15 bit
    operations; those who care to can use either.

    The *real* problem, and the usual cause for most postings here and in
    other forums, are that image processing newbies find the choice
    confusing: They don't know when or why to use one or the other. THAT is
    the problem.

    There is no "answer" to the question "which is better? 8 or 16?" It's
    the same kind of useless timewaster as all the other Usenet perennials
    (vi or emacs? Linux or BSD? MySQL or PostgreSQL?) I can't believe busy
    and intelligent people put energy into trying to answer unanswerables.
    As always: "use what works for you" and "it depends what you're trying
    to do".

    --T
     
    toby, Oct 27, 2005
    #3
  4. Bill Hilton

    Mike Russell Guest

    This is an interesting example, but why didn't Deke McClelland start with a
    normal photograph instead of one that has been heavily modified? I suggest
    it is because no such photograph exists.

    The article demonstrates that 16 bits can represent more brightness levels
    than 8. It does not address the issue of whether any actual photograph
    looks better after editing in 16 bits per channel than in 8 bits.
    Many minds are made up, and I respect those who have determined that 16 bits
    per channel, or 32 bits, is needed for them to achieve the desired result.
    I believe this issue is worth revisiting every once in a while, particularly
    when it is discussed constructively as has been the case here.
    Yes, it is a radical levels move that, in effect, closes the image like an
    accordion enough to cause banding in 8 bit, but not 16 bit. The same sort
    of move, just a more extreme, could be used to show that 24 bits per channel
    is better for editing than 16, or that HDR (32 bits per channel) is better
    than 24. In each case, nothing practical is being demonstrated except that
    more bits equals more levels.

    Once again, a qualification. I'm referring to color images in one of the
    customary Photoshop color spaces, specifically not a wide gamut space such
    as ProPhoto RGB. For those sorts of images, 8 bits per channel has plenty
    of head room even for extreme edits, and I have not yet seen such an image
    that shows otherwise.
     
    Mike Russell, Oct 30, 2005
    #4
  5. Bill Hilton

    toby Guest

    Dithering the output of the first adjustment would help a lot here, in
    8 bit. Does Photoshop not do this?

    But this is a silly example, since anyone with half a clue would
    understand that they had thrown away most of the significant bits by
    collapsing Levels in the first place (that can't be retrieved by
    expanding it again).

    --T
     
    toby, Oct 30, 2005
    #5
  6. Bill Hilton

    Mike Russell Guest

    [re banding showing up after a levels operation]
    Afaik there is no the only cases where Photoshop dithers is in gradients,
    profile conversions, and conversion from 16 to 8 bit mode. The first two
    are optional, and the third is always done. BTW - "dithering" is the adding
    of a typically small random value to break up banding patterns.

    [re article discussing editing in 16 bits]
    True enough. The scenario is unrealistic, and once again an artificial
    example - in this case a highly compressed photograph - is used to
    illustrate the superiority of editing in hibit.

    It's worth mentioning that there are any number of situations, ones that do
    not involve editing, where more than 8 bits per channel is useful. For
    instance a raw file may be underexposed several stops and still yield a
    relatively clean image. Raw files (which are linear gamma, BTW) can provide
    a handy way to multiply your ISO number by 8. If the same image is captured
    in 8 bits, such recovery is not possible.
     
    Mike Russell, Oct 31, 2005
    #6
  7. Bill Hilton

    toby Guest

    Yes, a good feature. Before they did that, we used to have to do it by
    hand: Make gradient over full range, add appropriate uniform noise,
    then shrink levels to final range... The only way to avoid banding in
    small-range gradients. There was no corresponding workaround for
    banding in strong blurs (and I guess there still isn't).
    ....or error diffusion, which is arguably even more effective, since the
    resulting noise is subjectively less obvious. Looks like Photoshop
    could do with an option to dither adjustments, at least.
    Yes, I could add that anyone who did not realise they were throwing
    away data in 8 bits, would not make the connection that "16"-bit mode
    would help here.
     
    toby, Oct 31, 2005
    #7
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