PS CS2 compression

Discussion in 'Digital Photography' started by Dave, Jan 5, 2007.

  1. Dave

    Dave Guest

    hello,

    when i down load a large jpg from my rebel xt the file size is around
    2.x mb. In PS, if i save it on the 12 setting the file size gets
    larger. What is happening to make a jpg bigger and is there any benefit
    to the pic? Also in raw (from the camera) it is 8.x mb, and ps raw
    gives the option to save as 25mb+. Same question, what is the software
    doing to make a small file bigger and is there any benefit?

    thanks

    ps: any words why the jpgs are so small from the rebel xt. I had a
    older canon that the jpg file sizes were twice as big. when I purchased
    the xt i figured with the 8 meg sensor the jpgs would have been bigger.
     
    Dave, Jan 5, 2007
    #1
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  2. Dave

    ray Guest

    On Thu, 04 Jan 2007 18:31:31 -0600, Dave wrote:

    > hello,
    >
    > when i down load a large jpg from my rebel xt the file size is around
    > 2.x mb. In PS, if i save it on the 12 setting the file size gets
    > larger. What is happening to make a jpg bigger and is there any benefit
    > to the pic?


    It is being saved with a higher 'quality' that it was saved with before.
    There is no benefit because lost data cannot be retrieved.

    >Also in raw (from the camera) it is 8.x mb, and ps raw
    > gives the option to save as 25mb+. Same question, what is the software
    > doing to make a small file bigger and is there any benefit?


    On my Kodak P850 (5mp EVF) raw files are 8mb, because the raw data is
    compressed - a tiff is 15mb. I expect you are seeing something similar. If
    the raw data were uncompressed, I'd be expecting about 22mb - 4mp x 4.5
    bytes/pixel since raw data is usually 12 bits per channel i.e. 4.5 bytes.

    >
    > thanks
    >
    > ps: any words why the jpgs are so small from the rebel xt. I had a
    > older canon that the jpg file sizes were twice as big. when I purchased
    > the xt i figured with the 8 meg sensor the jpgs would have been bigger.


    Probably more compression. Do you have the picture quality set to 'fine'?
     
    ray, Jan 5, 2007
    #2
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  3. In article <> ray <> writes:
    $On Thu, 04 Jan 2007 18:31:31 -0600, Dave wrote:
    $>Also in raw (from the camera) it is 8.x mb, and ps raw
    $> gives the option to save as 25mb+. Same question, what is the software
    $> doing to make a small file bigger and is there any benefit?
    $
    $On my Kodak P850 (5mp EVF) raw files are 8mb, because the raw data is
    $compressed - a tiff is 15mb. I expect you are seeing something similar. If
    $the raw data were uncompressed, I'd be expecting about 22mb - 4mp x 4.5
    $bytes/pixel since raw data is usually 12 bits per channel i.e. 4.5 bytes.

    12 bits per channel is 1.5 bytes. Don't forget that a RAW file
    has data right off the sensor, and each photosite on the sensor
    works with only *one* colour, not *three*. So for an 8 megapixel
    camera, that's about 12 megabytes for an uncompressed RAW file.
    Canon's RAW files are losslessly compressed, so they're of
    variable size (depending on how well the data from a particular
    image compress; this also explains why higher ISOs result in larger
    RAW files, as the extra noise doesn't compress well) and typically
    modestly smaller than the 12 megabytes of information they
    contain. 7-9 megabytes would be typical sizes for a Rebel XT's RAW
    files.

    Converting the RAW file applies Bayer interpolation, resulting
    in three colours per pixel - so for an 8 megapixel image, that's
    three bytes per pixel if you're working at 8 bits, yielding an
    uncompressed file size of around 24 megabytes.
    --
    Stephen M. Dunn <>
    >>>----------------> http://www.stevedunn.ca/ <----------------<<<

    ------------------------------------------------------------------
    Say hi to my cat -- http://www.stevedunn.ca/photos/toby/
     
    Stephen M. Dunn, Jan 5, 2007
    #3
  4. Dave

    acl Guest

    ray wrote:
    > On my Kodak P850 (5mp EVF) raw files are 8mb, because the raw data is
    > compressed - a tiff is 15mb. I expect you are seeing something similar. If
    > the raw data were uncompressed, I'd be expecting about 22mb - 4mp x 4.5
    > bytes/pixel since raw data is usually 12 bits per channel i.e. 4.5 bytes.
    >


    Hello. Raw data is indeed 12 bits per channel, but in fact each pixel
    only contains information about one channel: thus, 12 bits=1.5 bytes
    per pixel, or 7.5mb for an uncompressed file from a 5mp camera (and a
    bit more for an embedded jpeg).

    The reason there is information about only one channel per pixel is
    because it works like this:
    http://www.cambridgeincolour.com/tutorials/sensors.htm
    (I must admit to not having read it, but the explanations on that site
    are usually very careful). In short, there is a filter over the sensor
    that allows only one of R, G or B colours to pass for each pixel. Thus
    the colour resolution is lower than the luminance resolution (and in
    fact there are twice as many green as red or blue detectors), but this
    isn't a problem (convert a photograph in photoshop to LAB mode and
    smooth the L, then one of the other two channels, and see how much
    difference there is).

    Anyway. Hope this helps.
     
    acl, Jan 5, 2007
    #4
  5. Dave

    ray Guest

    On Thu, 04 Jan 2007 19:03:45 -0800, acl wrote:

    > ray wrote:
    >> On my Kodak P850 (5mp EVF) raw files are 8mb, because the raw data is
    >> compressed - a tiff is 15mb. I expect you are seeing something similar. If
    >> the raw data were uncompressed, I'd be expecting about 22mb - 4mp x 4.5
    >> bytes/pixel since raw data is usually 12 bits per channel i.e. 4.5 bytes.
    >>

    >
    > Hello. Raw data is indeed 12 bits per channel, but in fact each pixel
    > only contains information about one channel: thus, 12 bits=1.5 bytes
    > per pixel, or 7.5mb for an uncompressed file from a 5mp camera (and a
    > bit more for an embedded jpeg).
    >
    > The reason there is information about only one channel per pixel is
    > because it works like this:
    > http://www.cambridgeincolour.com/tutorials/sensors.htm
    > (I must admit to not having read it, but the explanations on that site
    > are usually very careful). In short, there is a filter over the sensor
    > that allows only one of R, G or B colours to pass for each pixel. Thus
    > the colour resolution is lower than the luminance resolution (and in
    > fact there are twice as many green as red or blue detectors), but this
    > isn't a problem (convert a photograph in photoshop to LAB mode and
    > smooth the L, then one of the other two channels, and see how much
    > difference there is).
    >
    > Anyway. Hope this helps.


    Thanks to both of you for the clarification.
     
    ray, Jan 5, 2007
    #5
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