Best Image -- Image Size vs Compression

Discussion in 'Digital Photography' started by john chapman, Aug 4, 2004.

  1. john chapman

    john chapman Guest

    I use a slide show package that will display images at full screen,
    regardless of the size/resolution that the monitor is set at. Since
    the slide shows, which are run from a CD, contain many images, there
    is a need to keep the image file sizes below 600KB or so. The images
    are edited in PS, and stored using PS's jpg save. PS can save jpgs at
    MAX (levels 10, 11, and 12) or HIGH (7, 8, or 9).

    What I am seeking is to get the most sharpness in the images while
    keeping the file sizes below 600KB. The trade-off I am looking at is
    a larger image at higher compression, or slightly smaller image at
    lower compression. On my test image I came up with the following
    combinations that fall within my desired file size range:

    1200 at 10 437KB
    1000 at 11 476KB
    800 at 12 494KB
    1100 at 11 572KB

    where, for example, 1200 is size of longest side, and 10 is PS
    compression setting

    In closely examining the images on the screen, it appeared to me that
    the 1100 @ 11 was the best, while the 800 @ 12 was the least sharp.
    Is there some known truth about size vs compression in terms of jpg
    quality, or does one simply arrive at some general truth by this kind
    of testing.

    Does anyone have any alternatives to recommend? Thanks in advance.
    john chapman, Aug 4, 2004
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  2. john chapman

    Ron Hunter Guest

    You will find that compression ratios are strongly dependent on the
    actual image data. Try compressing a picture with a lot of grass and
    trees and you will see the effect. Unfortunately, there is no answer
    that will work for all pictures. You will just about have to do each
    one on its own merits to get the best results. Frankly, I would match
    the compression to the original image size, and compress less if the
    picture is filled with complex shapes, such as grass and trees, and more
    if it is large areas of color. You may see more artifacts, but they
    will be less distracting.
    Ron Hunter, Aug 4, 2004
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  3. john chapman

    Martin Brown Guest

    A single test image will lead you seriously astray unless by chance you
    happened to have picked an unusually representative one.

    Do the test on a directory of about 100 images and you will get a better
    idea of how the size quality trade off works with a range of images.

    The ones to inspect really carefully are a couple of typical ones and
    the two with the most extreme smallest and largest file sizes after
    You would be well advised to chose a size that is nicely commensurate
    with typical screen sizes of 800x600, 1024x768, 1280x960, 1440x1080,
    1600x1200. Some display drivers can be relied upon to make a hash of
    displaying an image full screen with unusual sizes like e.g. 1100x825.
    You would have a lot more control of the compression settings with
    something based on the JIG codec. IrfanView springs to mind as
    reasonable quality free software with excellent batch rescaling and JPEG
    encoding functions.

    In addition it will not store a small essay on PhotoShop and various
    assorted colour management dross which will probably save you around
    50kb per file.

    Martin Brown, Aug 5, 2004
  4. Why not downsize to the resolution of the media first? In general,
    you can downsize a Bayer image (most cameras) to 25% of the original
    area with no optical loss whatsoever, since they are all digitally
    upscaled 400% as output (they have an interpolated value inserted
    between all their measured values, by a computer, after the shutter
    George Preddy, Aug 7, 2004
  5. What nonsense is this?

    Hans-Georg Michna, Aug 8, 2004
  6. john chapman

    john chapman Guest

    I do not understand the terminology. Regardless, my images are all
    scanned from slides and edited in Photoshop.
    john chapman, Aug 9, 2004
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