how to reduce the file size of an image

Discussion in 'Photoshop Tutorials' started by didi, Dec 20, 2003.

  1. didi

    didi Guest

    Hello all,

    Here is the deal. I have a pic that's 320x240 with a file size of
    255KB. I need it to be smaller than 62KB but with the same dimensions.
    How can I do that in Photoshop??

    Ty in advance.
     
    didi, Dec 20, 2003
    #1
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  2. didi

    jjs Guest

    Do you mean you want a 62kb image that is still 320x240? Or do you mean
    you want to retain the same aspect-ratio?
     
    jjs, Dec 20, 2003
    #2
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  3. didi

    Bill Hilton Guest

    From: (didi)
    File > Save for Web and try a jpeg compression setting of about 30-50 or so.
    Click '2-up' to see the compressed version side by side with the original to
    make sure you're not losing much quality. The estimated image size should be
    visible in the bottom left corner of the image window. Change the "quality"
    setting to increase/decrease file size and image quality until you get what you
    need, then save the file with a new name so you still have the original.

    Or you can do the same thing in ImageReady ... I can often get 650 x 480 jpegs
    down to 40-50 KB with good quality, so 320 x 240 in 62 KB should be pretty
    easy, you may be able to go with a setting of 70-80 or so.

    This will require some loss of data during jpeg compression, but you knew that,
    right?

    Bill
     
    Bill Hilton, Dec 20, 2003
    #3
  4. didi

    jjs Guest

    File > Save for Web and try a jpeg compression setting of about 30-50 or so.
    Click '2-up' to see the compressed version side by side with the original to
    make sure you're not losing much quality. [...][/QUOTE]

    Or Save for Web and use "Optimize to Filesize" (little arrow on the right
    of the window.) If you give it a crazy low value, it will bottom out at
    JPEG format with zero quality at the lowest possible size, not always your
    target.
     
    jjs, Dec 20, 2003
    #4
  5. didi

    Rowley Guest

    Is it a photo? If not then you might try saving it as a gif and reducing
    the number of colors in the image.
     
    Rowley, Dec 21, 2003
    #5
  6. The files size dictates the resolution/dimension ratio of an image. Reduce
    the file size and either the dimensions or the resolution have to be
    reduced. Below 72 dpi a picture will not 'look' normal on a PC screen. You
    figure out the rest, eh?
    Doug
     
    Techno Aussie, Dec 21, 2003
    #6
  7. The file size (on disc) is determined by the number of pixels in the image
    and, in the case of a compressed format such as JPEG, the degree of
    compression applied. The resolution setting (pixels per inch or PPI) has no
    effect whatever when the image is displayed in a web browser. The monitor
    display resolution setting will determine the image size. An image editing
    program can of course display the image at whatever size it chooses. The
    resolution setting in the image file will be used by print programs to
    determine the default print size of the image. The PPI setting of the image
    does not relate to the DPI setting of the printer.

    John
     
    John Houghton, Dec 21, 2003
    #7
  8. didi

    didi Guest

    Or Save for Web and use "Optimize to Filesize" (little arrow on the right
    of the window.) If you give it a crazy low value, it will bottom out at
    JPEG format with zero quality at the lowest possible size, not always your
    target.[/QUOTE]


    Thank you for the answers. I think I got confused. In photoshop when I
    was in "Image size" it was showing 255K. However in Windows explorer,
    when I was looking at the file size it was saying 14KB! So my file is
    already small enough in KB. But I still don't understand what the 255K
    refers to...
     
    didi, Dec 21, 2003
    #8
  9. The basic file size = no. of pixels x 3
    (i.e. one byte per channel per pixel).

    John
     
    John Houghton, Dec 21, 2003
    #9
  10. didi

    Tacit Guest

    Thank you for the answers. I think I got confused. In photoshop when I
    255K is the size of the image when it is UNCOMPRESSED. 14KB is the size of the
    image when it is COMPRESSED.

    GIF and JPEG files are compressed in order to make the file smaller on disk. In
    the case of JPEG, the file is made smaller by a process that degrades its
    quality. When you save an image as a JPEG, the image is small on disk, but its
    quality is degraded.

    When you open an image in Photoshop (or anything else), the image must be
    uncompressed in order to be shown on the screen. Photoshop shows you the
    uncompressed size of the image.
     
    Tacit, Dec 21, 2003
    #10
  11. didi

    Tacit Guest

    The files size dictates the resolution/dimension ratio of an image.

    That is incorrect. Many image formats, including GIF, JPEG, PSD, and some
    varieties of TIFF, are compressed on disk. The size of the file is NOT
    determined strictly by the size of the image in pixels.
    Incorrect. When an image is shown on the screen, the resolution is unimportant
    and the information about resolution is discarded. The computer displays the
    image at one image pixel per screen pixel. A 320x200-pixel image at 4 dpi will
    look *identical* to the same 320x240-pixel image at 1,000,000 dpi.
     
    Tacit, Dec 21, 2003
    #11
  12. There was a time when I thought you actually had a clue, Tacit... Time for a
    re-think on that area. You contridict yourself here. You say it's
    "incorrect" that the pixel count of an image dictates the size/resolution of
    the picture yet go on to say exactly that in different words. Sure you're
    not just testing your keyboard/mindset to see if what you say can be said
    any differently?

    Doug
    -----------------------
     
    Techno Aussie, Dec 22, 2003
    #12
  13. Sounds like you've become clue-deficient yourself. "dpi" is a completely
    arbitrary measurement that only takes on meaning when it is used to map the
    image to a physical substrate (i.e., paper) with a fixed dimension. You can
    assign any dpi you like to any digital image, from 1 dpi to 10,000,000 dpi,
    and it does not change the fixed pixel-count height and width. The 72 dpi
    or 96 dpi figures used for screen display are rough approximations of the
    physical mapping of pixels to screen inches. And that's almost exactly what
    Taccy said.

    So what's [the | your] problem here?
     
    James Gifford, Dec 22, 2003
    #13
  14. didi

    Bill Hilton Guest

    From: "Techno Aussie"
    Personally I think Tacit usually gives the best answers of anyone on the entire
    newsgroup, except maybe for some of the inkjet RGB color management-type
    questions.
    No, he said that "When an image is shown on the screen, the resolution is
    unimportant" ... resolution doesn't really matter until you're ready to print.
    He's not talking about the actual pixel dimensions, he's talking about the
    resolution, which is different.

    Bill
     
    Bill Hilton, Dec 22, 2003
    #14
  15. didi

    Tacit Guest

    There was a time when I thought you actually had a clue, Tacit... Time for
    No; "resolution" and "pixel dimension" mean two different things.

    Resolution, measured in pixels per inch or dots per inch, is a measurement of
    how many pixels occupy one inch of output; pixel dimention is a measure of the
    total number of pixels in an image.

    So: Pixel dimension means how many pixels there are; resolution measures how
    big each pixel is.

    It's easy to confuse the two. Think of a raster image as a tile mosaic.
    Resolution is how big each tile is; pixel dimension is how many tiles there
    are.

    Resolution, pixel dimension, and uncompressed file size are all related. If you
    take the number of pixels wide the image is, and multiply that by the number of
    pixels per inch, you'll have how many inches wide the image is...

    ....when it's printed. That's the confusing part. When an image is displayed on
    the screen, the resolution is unimportant, and is discarded. Only the pixel
    dimension matters.
     
    Tacit, Dec 22, 2003
    #15
  16. didi

    Warren Sarle Guest

    "Size" can mean (not an exhaustive list):
    1) the number of bytes in a file
    2) the number of pixels in an image
    3) the amount of paper occupied by an image

    "Resolution" can mean:
    1) the number of pixels per linear unit of paper
    2) the number of rows and/or columns of pixels in an image
    3) the number of distinguishable lines per linear unit of film
    4) the number of distinguishable lines per image

    If you are clear about which meaning you intend, you are
    less likely to be misunderstood.
     
    Warren Sarle, Dec 22, 2003
    #16
  17. didi

    Xalinai Guest

    For a file that small in pixels, 255 KB seems to be an uncompressed
    format (320*240*3=225KB) plus unnecessary information.

    Any JPG-compression should reduce it to less than a third, I'd start
    at minimum compression - and in Photoshop you should "save for web" to
    avoid any additional stuff in the file.

    Michael
     
    Xalinai, Dec 23, 2003
    #17
  18. Seemingly you too, fail to realise that the pixel count controls the image's
    size/resolution. You can't "asign" dots that don't exist anymore than you
    can pixels that don't exist. If an image is 1024 pixels by 768 pixels in
    size, it's at 72 dpi to display on a computer screen. Double the dpi count
    to get a basic quality print, and you halve the actual image size. Unless
    you are talking about interpolation or other methods of altering the pixel
    count.
    Doug
     
    Techno Aussie, Dec 23, 2003
    #18
  19. didi

    Tacit Guest

    Seemingly you too, fail to realise that the pixel count controls the image's
    No. Pixel count controls size, but not resolution.

    Consider two images: one 320x240 at 72 pixels per inch, one 320x240 at 300
    pixels per inch. Same pixel count, different resolution. You are using "pixel
    count" and "resolution" as if they mean the same thing. They don't.
     
    Tacit, Dec 23, 2003
    #19
  20. Unfortunately, they can be. The term 'resolution' is also commonly used
    for the pixel count of digital cameras. I don't like it either, but
    that's the way it is.
     
    Johan W. Elzenga, Dec 23, 2003
    #20
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