Page 1 of 3 123 LastLast
Results 1 to 10 of 21

Thread: Converting an 8-to 16-bit grayscale before adjusting ?

  1. #1

    Join Date
    Sep 2003
    Location
    Massachusetts USA
    Posts
    7,797

    Converting an 8-to 16-bit grayscale before adjusting ?

    I always presumed that if we convert an 8-bit grayscale image to 16 bit before performing any adjustments, Photoshop will "fill-in" the gaps in the tonal scale as adjustments are made. Now I wonder if this is true.

    The histogram tool is a bit small for me to determine the nature of the file. Is there a better tool ?

  2. #2

    Join Date
    Sep 2003
    Location
    Massachusetts USA
    Posts
    7,797

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    I did an experiment where I converted an 8-bit grayscale image to 16-bit, then added adjustments. The histogram continued to show banding. I saved the image, and the histogram still showed banding.

    Then, I re-sized the image ever so slightly: that caused the banding in the histogram to go away. I presume that Photoshop reworked the tonal scale at that point.

  3. #3

    Join Date
    Oct 2015
    Posts
    363

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    Ken,

    AFAIK converting an 8-bit file to 16-bit can be beneficial because PS now performs its computations at that level. However, 8-bit is and will be 256 levels even when converted to 16-bit; no data will be magically added. Therefore, any destructive editing operation will still be removing data from a file that didn't start with much. Best to start with a 16-bit file, edit at that level, then repurpose for output.

  4. #4

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    The ONLY time it's ever beneficial to convert to 16 from 8 bits is where you are generating gradations or new skies from scratch. And you still have to add a small amount of noise even in 16 bit mode. Simply converting to 16 bit just spreads the existing data out further but can't add any real detail to your image, it only puts an 8 bit file in a 16 bit container. The slight resample - and you do have to resample and not just resize - will indeed smooth out a histogram, but it won't actually have any beneficial impact on real posterization in the image. You have to add noise for that.

  5. #5
    8x10, 5x7, 4x5, et al Leigh's Avatar
    Join Date
    Dec 2010
    Location
    Maryland, USA
    Posts
    4,992

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    If you convert an 8-bit file to 16 bits, you still have 256 distinct colors, no more.

    If you use a color slider on a pixel AFTER the conversion, that adjustment will be made on the 16-bit data word.
    But only on that one pixel.

    If you select multiple pixels of the same color simultaneously, the adjustment would be for those only.

    If you adjust multiple pixels having different colors simultaneously, the result will depend on the software.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  6. #6
    Kirk Gittings's Avatar
    Join Date
    Mar 2004
    Location
    Albuquerque, Nuevo Mexico
    Posts
    9,712

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    We used to discuss this about using older scanners which could only deliver 8 bit files. There was some consensus, if I remember right, that there was some benefit to changing it to 16 bit immediately after the scan and before doing work in PS, but not as good as just scanning in 16 bit. I did this for a few years on B&W files because I had 8 bit scanners. I "believed" there was a benefit, but don't remember seeing or attempting any critical comparison.
    Thanks,
    Kirk

    at age 67
    "The woods are lovely, dark and deep,
    But I have promises to keep,
    And miles to go before I sleep,
    And miles to go before I sleep"

  7. #7

    Join Date
    Sep 2003
    Location
    Massachusetts USA
    Posts
    7,797

    Re: Converting an 8-to 16-bit grayscale before adjusting ?







    In my humble test, the histogram was smoothed out only after resizing the image: I changed the size by 1 pixel.

    Apparently the process of resizing the whole image causes intermediate tones to be re-computed.

    This test relies on the histogram tool, which may be deceptive. If it's a reliable indicator, it suggests that after we import an 8-bit file we should convert to 16-bit and resize it - however modestly - before performing any adjustments.

  8. #8

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    "In my humble test, the histogram was smoothed out only after resizing the image: I changed the size by 1 pixel.

    Apparently the process of resizing the whole image causes intermediate tones to be re-computed.

    This test relies on the histogram tool, which may be deceptive. If it's a reliable indicator, it suggests that after we import an 8-bit file we should convert to 16-bit and resize it - however modestly - before performing any adjustments."

    You've pretty much proven to yourself that your own test is invalid. What's the point here anyway? First of all, you're obsessing about histograms, which, while a useful tool on the back of a digital camera or during the initial scanning stage on a good scanner, don't tell you much of anything about the quality of an image, being NOTHING more than a simple graph of the distribution of pixel values in an image or a selection within an image. The only thing you've shown is that by re-sampling, you close the gaps in a histogram and you seem surprised by this. Of course it will, but it won't actually smooth over visible posterization in the image. Adding noise to the image will smooth out a histogram too but will not necessarily make a better looking image except where you need a small amount to smooth gradations.

    People used to get excited about histograms twenty years ago and a certain color management "expert" really obsessed over them, and almost always losing sight of the real final goal, which has nothing to do with the histogram, well, unless you're into printing histograms.

  9. #9
    jp's Avatar
    Join Date
    May 2009
    Location
    Maine
    Posts
    4,491

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    In CC, I can resize the adjustment histogram to be bigger in a curves layer by dragging the left edge of the popout box even more to the left. I have not tried resizing the histogram in the upper right that's part of the photography workspace.

    I bet there are other things you can do to apply the 16bitness. Perhaps adding a layer and resaving?

    Sas; Ken is a master of midtones and of course the histograms would be important. I think they are important because it's how I make good tones in B&W scans which I scan sorta flat on purpose and tune in photoshop.

  10. #10

    Join Date
    Sep 2003
    Location
    Massachusetts USA
    Posts
    7,797

    Re: Converting an 8-to 16-bit grayscale before adjusting ?

    Sorry for any confusion: occasionally I use a digital point and shoot camera - or an iPhone - and just couldn't figure out how to convert the image to a more lossless bit-depth.

    I found one method. As JP points out, there are probably many.

Similar Threads

  1. Grayscale Gamma 2.2 & QTR
    By DHodson in forum Digital Processing
    Replies: 13
    Last Post: 1-Jul-2016, 20:12
  2. scanning at 16 bit grayscale with 4990
    By Jeff Graves in forum Digital Processing
    Replies: 23
    Last Post: 17-Jan-2011, 11:28
  3. Convert Color to Grayscale
    By sanking in forum Digital Processing
    Replies: 20
    Last Post: 14-Feb-2008, 15:57
  4. update on RGB vs. Grayscale scanning
    By Dave Aharonian in forum Digital Hardware
    Replies: 7
    Last Post: 21-Jun-2005, 09:36
  5. 200 lpi grayscale instead of steptablet
    By Martin Kapostas in forum Darkroom: Film, Processing & Printing
    Replies: 3
    Last Post: 22-Oct-2001, 06:19

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •