Technical question about image scaling

Discussion in 'Digital Cameras' started by Peabody, Mar 27, 2009.

  1. Peabody

    Peabody Guest

    As will be obvious, I'm new at this, but just want to
    understand.

    So everybody says optical zoom is good and digital zoom is
    bad, and in general I understand why. But my Canon A590 has
    something called "Digital Tele-Converter" mode, which simply
    fixes the amount of digital zoom at 1.6x or 2.0x, and you
    can add whatever optical zoom you want, if any, on top of
    that. I think this means the camera only uses a smaller
    central segment of the sensor as its "raw" data, and
    produces the final jpeg from that.

    If the final jpeg size is bigger than that central segment,
    then the camera would have to upscale the raw image to get
    the final jpeg, and that's where the "digital zoom is bad"
    thing comes from because there's going to be a loss of image
    quality.

    But suppose you're shooting in one of the small jpeg sizes
    anyway. In that case, the use of the digital tele-converter
    mode means that you're just doing a more modest downscaling
    than you would otherwise do.

    So in my mind this raises the theoretical question of
    whether this modest downscaling should produce any worse
    result than downscaling from the full sensor raw data. Is
    there any way to generalize about that?

    It does seem to me that there might be good reason to use
    the digital tele-converter mode for smaller jpegs if there's
    no quality penalty, and that would be that you can "zoom in"
    without changing the effective aperture as would happen when
    using optical zoom. Well, for example:

    If I can ever find an artist to participate, my next project
    is a time-lapse video of a painting being painted. Since
    there are going to be thousands of individual images, and
    since it's going to end up as a video anyway, I'm probably
    going to be shooting in 2MP mode - 1600x1200. Well, it
    just seems that the digital tele-converter mode might
    actually be better than optical zoom. Using some form of
    zoom, the camera could be farther away from the artist, and
    therefore less audible or in the way, but using digital
    tele-converter mode I could avoid the reduction of aperture
    that optical zoom would produce. There would be no need to
    change shutter speed or ISO to compensate. I'm not sure
    about the effect on DOF of the two zoom options.

    Anyway, bascially you have two situations:

    1. 2x optical zoom, no digital zoom, and the full raw image is
    downsized from 8mp to 2mp.

    2. no optical zoom, fixed 2x digital zoom, and that smaller
    raw image is also downsized to 2mp.

    The question is - can you say anything in general about the
    relative quality of the final jpeg between these two
    situations? Is one always better, or does it depend? On
    what?

    Thanks very much.
     
    Peabody, Mar 27, 2009
    #1
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  2. Peabody

    Jay Kneese Guest

    Smaller images are created by "skipping" rows and columns of pixels when
    shifting the sensor data to the buffer or memory card. Obviously, a digital
    zoom from an already degraded image will be even worse. Think about it.
     
    Jay Kneese, Mar 27, 2009
    #2
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  3. Those are two totally different animals. Bigger JPEG and smaller JPEG
    have nothing to do with zoom or focal length, be it digital or optical.
    JPEG supports different compression rates and bigger JPEG simply means
    less compression and thus less information loss and better picture
    quality.

    jue
     
    Jürgen Exner, Mar 27, 2009
    #3
  4. Peabody

    Don Stauffer Guest

    One reason many of us do not think much of digital zoom for serious work
    is that whatever the camera does as digital zoom, you can do in an image
    editing program, and often do it better.

    Many of the editing programs give you choices of the mathematical
    algorithm you use if you do have to downsample.

    And, doing it in your camera is essentially "reversible" as long as you
    save the downsampled image as a different filename.

    Digital zoom is for those folks who do not use computers with their
    digital camera, and even so, optical zoom is generally better.
     
    Don Stauffer, Mar 28, 2009
    #4
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