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No. of pixels vs file size

Discussion in 'Digital Photography & Imaging' started by gurky, Oct 10, 2013.

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  1. gurky

    gurky Thread Starter

    Apr 14, 2012
    I process JPGs from two different cameras. I use the Linux command "mogrify"
    to reduce the size to 70 kb for thumbnails. However, those from one camera
    end up with fewer pixels than from the other, so that the thumbnails are blurred,
    even though the size in kb is the same in both cases.

    Why is the file size not proportional to the product of the pixel size in x and y,
    i.e. the total number of pixels?
  2. cwwozniak

    cwwozniak Trusted Advisor Spam Fighter

    Nov 28, 2005
    First Name:
    The more variation there is between adjacent pixels in the original image, the larger the resulting file after JPEG compression is applied. If you are producing JPEG thumbnails of a certain file size, a simple image can have larger x,y dimensions than a complex image.
  3. Peano

    Peano Banned

    Feb 10, 2006
    Terminology: image size and file size.

    Image size is measured in pixels --- 1024 x 800 (for example)

    File size is measures in kilobytes or megabytes -- 140K, 2.4 MB

    Two variables that affect file size:

    1. Details in the image. More details increases file size (because there's more image detail to store).

    2. Quality level in saving. Higher quality level increases file size..

    If you're going to save a large image as a thumbnail, first reduce the image size. If the original image is, say, 3000 x 3000 and you need a thumbnail that is 100 x 100, reduce image dimensions first and save the thumbnail at a reasonable high quality level. In Photoshop, for instance, use quality level 10. Then check the file size of the thumbnail. If it's too large, save at a lower quality level.
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