ACR 16 bit raw photo conversion

Discussion in 'Digital SLR' started by Paul Furman, Apr 14, 2006.

  1. Paul Furman

    Paul Furman Guest

    [x-posted to alt.graphics.photoshop & rec.photo.digital.slr-systems]

    I discovered (perhaps misguided?) that if I zero out all the adjustments
    in ACR (Adobe Camera Raw) except white balance, and save as 16 bit, the
    result has enough information to do anything that ACR will do for
    contrast, saturation, etc. I'm using CS (not CS2).

    For high contrast images, I used to do two conversions for highlights &
    shadows & mask those together but it can be done with one straight 16
    bit conversion & masking adjustment layers with a lot more flexibility.

    I had thought that the ACR adjustments were working on some magical RAW
    level of high bit math but in fact, 16 bit (vs 8 bit) holds everything
    there is to get. I'm familiar with holding down the alt key in ACR while
    sliding exposure and shadows and that's a nifty tool but why didn't they
    just put it in the core PS adjustments and why these new terms like
    shadows & brightness?

    On a related note, ImagePlus http://www.mlunsold.com/ is a program used
    for astrophotography which features a higher bit math, which is reported
    to perform cleaner conversions.

    From their descriptions:
    "- Read and write 8, 16, and 32 bit integer or 32 bit float RAW images
    with one or 3 color channels. Interleaved or non-interleaved channels
    are also an option.
    - Read and write 8, 16, and 32 bit integer or 32 bit float and 64 bit
    double FITS image formats."

    The argument I heard is photoshop *only* does 15 bit math and this does
    64 bit math so there is less rounding error and cleaner results,
    especially in the shadows where the amount of information is minimal.
    Then I hear that camera files are really only have 12 bits so 15 should
    be plenty, right? I'm not clear what 32 bit or 64 bit even means.

    One issue may be that RAW data is linear, not gamma adjusted, so when
    you do the gamma adjustment to 2.2 that stretches the data compressing
    highlights and expanding shadows but expanding won't damage anything and
    the highlights can usually afford to be compressed some. Strictly
    speaking this expands from 4096 levels to about 13000 levels or about 3x
    if you don't do any compression.

    I've also read that photoshop has limitations to what can be done with
    high bit images such as 16 bit & 32 bit like no adjustment layers. I
    think this is outdated and not an issue in CS. I don't see any option
    for 32 bit files though.

    Any insights on these isssues?
     
    Paul Furman, Apr 14, 2006
    #1
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  2. Paul Furman

    Mike Russell Guest

    IMHO 8 bits has plenty of headroom for editing, and I have yet to see any
    exceptions to this in images using "normal" colorspaces.

    That said, it's a fairly complex issue, and reasonable people differ. The
    question is becoming moot as processors get faster and memory gets cheaper.
    Here are some more insights that you might find interesting.

    http://mike.russell-home.net/tmp/DanMargulis16Bit/
     
    Mike Russell, Apr 14, 2006
    #2
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  3. Paul Furman

    Jeremy Nixon Guest

    Well, you're doing two different things. When you make the adjustments in
    ACR, you're controlling the mapping of the raw data into the final image;
    in PS proper, you're adjusting the final image. The result may be visually
    the same, though, in which case, I guess you just do it whichever way you
    prefer. :)
    I found that any need to do this was removed by the inclusion of Curves
    in Camera Raw.
    It's not a magic level of higher-bit math, it's just a different operation.
    I suspect the terms were chosen to make sense to photographers.

    What you're doing with those sliders is adjusting the mapping of the linear
    raw data into the gamma-encoded image. Clipping happens when something ends
    up getting mapped outside the valid range. There isn't quite the same
    concept when working on image data in Photoshop itself. So using different
    terms does make sense on that level.
    Looks Windows-only. Shame.
    The raw files are 12-bit linear. Taking that into a gamma-encoded image
    without loss will require more than 12 bits. Whether real 16-bit as opposed
    to what Photoshop does would be better, given current input camera data and
    what we tend to do with it, I don't know, but it sure couldn't hurt.
    Current versions can do things like adjustment layers on 16-bit images.
     
    Jeremy Nixon, Apr 14, 2006
    #3
  4. I'm not sure I fully understand what you're asking, but one thing I wanted
    to clear up about ACR and RAW in general ...

    When you make adjustments in ACR, you're not adjusting the image per sec -
    what you're actually doing is customising the conversion - the net result of
    which is a different image. The difference is subtle but important.

    If you're doing most of your adjustments in photoshop then you're adjusting
    an image that's already been adjusted, and there can be a price to pay: If
    they're relatively minor corrections it doesn't usually make a lot of
    difference (in 8 or 16 bit modes) - the difference is most apparent in
    situations where you need to make large adjustments and only have 8 bits to
    work with - in which case doing them in ACR gives you a big head start.
     
    Pete Mitchell, Apr 14, 2006
    #4
  5. Happens all the time :)
     
    Pete Mitchell, Apr 14, 2006
    #5
  6. Paul Furman

    Mike Russell Guest

    Got Image?
     
    Mike Russell, Apr 14, 2006
    #6
  7. Took you a whole 3 minutes - you're getting slow in your old age Mike :)
     
    Pete Mitchell, Apr 14, 2006
    #7
  8. Paul Furman

    Mike Russell Guest

    LOL. I'm here for ya. So no image, then :-(
     
    Mike Russell, Apr 14, 2006
    #8
  9. I try to expose mine correctly in the first place :) and my Bruce + Chris +
    Fred weigh more than your Dan!
     
    Pete Mitchell, Apr 14, 2006
    #9
  10. Paul Furman

    Paul Furman Guest

    Yeah sounds like a nice upgrade.

    Sort of a dumbed down version? I had to re-learn how to work with those
    sliders now I find out they were's doing anything special? If true, I
    wish I'd known.

    So maybe this is the magic? Maybe it does the adjustment before gamma
    encoding?
     
    Paul Furman, Apr 14, 2006
    #10
  11. Paul Furman

    Jeremy Nixon Guest

    "Exposure" sets the white point. "Shadows" sets the black point.
    "Brightness" is a gamma adjustment.
    It does, yes. Even the Curves are applied before the gamma adjustment,
    though the UI is cleverly crafted to *appear* to be working on post-gamma
    image data.
     
    Jeremy Nixon, Apr 14, 2006
    #11
  12. Paul Furman

    Paul Furman Guest

    What you say below is what I've been taught but I'm not sure it's true.
    Adjusting in ACR clips the image so that further adjustment is hampered.
    That's fine if it comes out right but if no adjustments are made in ACR,
    you retain full control in PS.

    Note that an ACR conversion with all adjustments zeroed out is quite
    dark and needs heavy adjustment.
     
    Paul Furman, Apr 14, 2006
    #12
  13. My understanding of it is that the difference is in whether or not the these
    parameters are adjusted before (as in ACR) or after (as in PS) the gamma is
    changed.

    I'm not sure what you'd get with everything zeroed - to be honest, the
    default ACR / RAW workflow works pretty well for me, so I don't spend a lot
    of time playing with any of the extremes.

    I'm sure JPS will chip in soon and steer me towards the true path :)
     
    Pete Mitchell, Apr 14, 2006
    #13
  14. Paul Furman

    Paul Furman Guest

    OK that makes sense.
    Confusing that they use different names though!
    I didn't hardly use levels before (just curves) so it was especially
    unfamiliar. It seems all that's needed is a curves tool with a gamma
    slider as an alternate means of shaping the curve.

    Hmm, OK, that sounds reasonable. The common knowlege about this seems
    more far-reaching than that though. My point about stretching shadows &
    compressing highlights stands; it shouldn't help at all with the shadows
    & shouldn't be noticeable in the highlights exept in very unusual
    extreme cases. I would be interested in seeing actual examples of a
    discernable difference: I don't mind if the example is grossly
    exaggerated and zoomed to 1600% I just haven't been able to see any
    difference. But with this in mind I'll probably try again.
     
    Paul Furman, Apr 14, 2006
    #14
  15. Paul Furman

    Paul Furman Guest

    It gives good highlight detail.
    I often want less contrast, more detail in the highlights and shadows.
    I've been happy with the ACR tools, it's just strange that they don't
    fit the existing terminology. It's so easy to mess things up when I
    don't know what I'm doing so I want to understand.

    I hope so, I think I exasperated him with that last thread :)
     
    Paul Furman, Apr 14, 2006
    #15
  16. Paul. have you seen this page?

    Digital Camera Raw Converter Shadow Detail and Image Editor Limitations:
    Factors in Getting Shadow Detail in Images
    http://www.clarkvision.com/imagedetail/raw.converter.shadow.detail

    Why raw converters have the problems shown on this page
    is baffling. With 12-bit camera data, 16-bits ought to
    be more than enough precision. But then its really 15-bits
    with some tricks to speed operations, and I think we are
    seeing the limitations of that math.

    I am finding more and more 15-bit processing problems from
    Photoshop's 15-bit math, and switching more and more
    to 64-bit ImagesPlus. I personally don't think 64-bit
    matters. Normal 16-bit processing should do just fine.
    Professionally, I process a lot of scientific imaging
    data in 16-bits and have not seen the same issues
    I see in photoshop. I was not fully aware of these
    problems until I did similar work in imagesplus for my
    astronomy photos, then work for this article:

    Image Restoration Using Adaptive Richardson-Lucy Iteration
    http://www.clarkvision.com/imagedetail/image-restoration1

    Look at Figure 7, and note the photoshop image (middle)
    looks like what I call pasty. The fine detail in the
    fox's hair above right of the eye looks smoothed out.
    The more I compare photoshop results with imagesplus,
    the more I see these problems. It is only an issue
    with very high signal-to-noise data as noise masks the
    problem.

    But imagsplus does not have the slick user interface
    of photoshop and simply doesn't do some things well at all
    (including curves). No feathered selections, no
    color management. But it does the math scientifically
    correctly, and some tools work better in my opinion
    (like levels).

    On my digital workflow page,
    http://www.clarkvision.com/photoinfo/digitalworkflow
    I do steps 1-3 in imagesplus (2 is part of the
    raw conversion), then steps 6 and 7. Others are
    in photoshop.

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 14, 2006
    #16
  17. Paul Furman

    Paul Furman Guest

    Alright, I was able to produce an obvious difference by pushing all the
    sliders to the maximum position then matching that appearance on a
    zeroed out 16 bit ACE tif with levels & contrast adjustments. It's
    possible the additional 'brightness & contrast' adjustment was more
    destructive than using only levels, I read that's more destructive somehow:

    http://www.edgehill.net/1/?SC=go.php&DIR=Misc/photography/raw-convert
    It's clear that the top version is much smoother.
    Granted this is *very* *very* extreme. I tried looking at some lightened
    shadow detail this way where nearly black was raised to middle gray &
    added contrast (much more than normal) and it was very difficult to find
    any difference, pixel to pixel at 1600 % zoom.
     
    Paul Furman, Apr 14, 2006
    #17
  18. Paul Furman

    Jeremy Nixon Guest

    Like I said, I suspect the names were chosen to make sense from a
    photography standpoint.

    I do think it's fair to use different names from what is used in Photoshop
    itself, though, since the controls are doing something different. Using
    "white point", "black point", etc., might make some sense from the image
    processing standpoint, but then you'd have people from the opposite side
    wondering what in the heck they mean.
    Technically, there's nothing you can't do in Curves that you can do with
    level controls. It's just two different interfaces to the same thing --
    forming the transfer curve. However, those Camera Raw sliders do have
    some "magic" behind them, so they make sense from a UI standpoint.

    For example, I don't think that Brightness is a strict gamma function.
    And, if you move it above 100, it can also affect clipping as it raises
    the "brightness" of the image beyond what you'd think of as a gammma
    adjustment. And "Contrast", whose purpose is to adjust midtone contrast,
    is definitely a bit smarter than a simple S-curve; I don't know what kind
    of curve it's creating, as it doesn't show you, but the result is
    different from going into Curves and applying a naive S-curve. Plus,
    Exposure has the additional "magic" of highlight recovery at negative
    settings.

    So, using plain technical names for the sliders wouldn't be correct,
    either, since they do a bit more than merely, for example, "setting the
    white point". Inventing a name for them is really the right way to go,
    from a UI standpoint, lest you confuse people who know what it means to
    set the white point and expect that nothing else happens as a result of
    adjusting that slider.

    Basically, the UI was created to make sense, not to reflect any simple
    technical terminology.
     
    Jeremy Nixon, Apr 14, 2006
    #18
  19. Paul Furman

    Paul Furman Guest

    Yes and that's a really dramatic example. I just started tinkering with
    the ImagePlus demo and haven't been able to do much yet.
    It may be my relatively noisy D70 (vs your 1DsMII) can't benefit from
    the high bit math but that page shows a special sharpening routine and
    that's separate from bit depth & raw conversion issue. Do you only see
    the benefit with low ISO images, or only in the less noisy mid tones and
    highlights?
    Thanks, I'll study that more while exploring ImagePlus.
     
    Paul Furman, Apr 14, 2006
    #19
  20. Paul Furman

    Siggy Guest

    In
    Wonderful read that, Mike. Many thanks! Wooohoooo. I feel L I B E R A T E D!
    Those 76Mb 16 bit ProPhotoRGB TIFFS are now history. :)

    Nigel
     
    Siggy, Apr 14, 2006
    #20
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