Pixels v Jpeg File Size v Print Size??

Discussion in 'Digital Cameras' started by PeterH, Jan 18, 2004.

  1. PeterH

    PeterH Guest

    New to digital photography and need to get it in my mind how pixels, file
    size and print size work.

    I have a Canon 300D (6MP) and prefer to take all shots at maximum resolution

    Example: I take a maximum resolution photo producing a 3MB jpeg file. Can I
    get a 6"x4" print using the full resolution of the 3MB file or does a 6"x4"
    only print at say 200K?

    In other words, does a print of any physical size only have a certain number
    of pixels per square inch? Therefore a 6x4 only ever uses/requires say a
    200KB file and the remaining 2.8MB is wasted or can the full 3MB be used to
    print a better detailed 6x4?


    PeterH, Jan 18, 2004
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  2. PeterH

    Trev Guest

    Where to start.
    Digital images are measured in Pixels, width "n" X height "n" will give
    the total in your case in mega pixels.
    To set a print size we pack those pixels together to a Sq inch to
    produce the ppi and a dimension in inches
    So 6 x 4 " at 300 ppi = 6 x 300=1800 pixels By 4 x 300 = 1200 pixels
    If you had picked 250 ppi there would have been less pixels and 400 ppi
    would have needed more pixels to produce the same size print.
    as You will have much more pixels then that you would need a higher ppi
    to get it to 6 x 4 inch. And as the aspect ratio is deferent you will
    need to crop a bit off.

    I have not mentioned file size as the makes no difference to printing.
    Each Pixel as 8 bits of red and 8 bits green plus 8 bits blue adding up
    to 24 bits per pixel multiply that by the amount of pixels width and
    multiply by height and you get the file size.
    Jpeg is a loosey file format the examines you image in blokes of 16 sq
    pixels 4 across and 4 down. it looks for similarities in the pixel makes
    a note of there position saves one as a reference and discards the rest.
    depending on your settings just how close the similarity has to be to be
    saved. This will make the file size smaller for storage or
    transportation be cause the are less of those 24 bit pixels. When that
    file is opened, those missing pixels a recreated based on the saved one
    and notes as to the location of where similar ones should be placed.
    Its good but not perfect.
    Trev, Jan 18, 2004
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  3. PeterH

    Jeffrey Guest

    It really depends upon the maxixum effective resolution your printer is
    capable of producing (not to be confused about how many dots per inch). Old
    style 3 or 4 colour inject printers, had to do a lot of dithering (ie mixing
    a lot of different colour dots in patterns) to simulate different colours,
    so you needed a lot of dots to make an effective pixel. These printers may
    have only acheived say 72 effective dots/pixels per inch.

    Newer printeres these days, have more colours (HP7960 has 8), have variable
    size drops, and shoot more than colour on the same dot. These printers can
    therefore have a much greater effective resoluton, probably greather than
    300 but less han 600 effective dpi.

    Therefore, if you intending to print your 6x4's on an older style printer,
    you would only need

    6in x 72 dpi *4in *72dpi * 3bytes (24bit colour) =

    432 x 288 x 3 = 373248 bytes (say 200K JPEG with compression).

    However, with a new printer (lets say 300 effective dpi) you will need

    6in x 300 dpi *4in * 300 dpi * 3bytes (24bit colour) =
    1800 x 1200 x 3 = 6,480,000 bytes.

    Compared to the max resultion of your camera

    3072 x 2048 x 3 = 18,874,368 bytes.

    I also have a 300D, and after obtaining that camera, I bought the HP7260 for
    my photoprinting, because my old Epson 760 was no longer good enough (but I
    still use it for text printing).

    When I am print 6x4 prints, I take advantage of the extra resolution, to
    crop the photo, so that I only print out that part of the photo that I want
    to print. I still give up a fair bit of the resolution available to me, but
    I still produces a great result.


    PS. I have unfortunately not been able to use the cropping feature in
    Photoshop Elements Print Preview, to do the cropping.
    Jeffrey, Jan 18, 2004
  4. PeterH

    PeterH Guest

    Thanks to all who replied - all excellent answers.

    I will read your posts a couple of times to be sure I have it right, but
    it's making a lot more sense now.


    PeterH, Jan 18, 2004
  5. PeterH

    Don Stauffer Guest

    The image inherently has a given number of pixels, regardless of whether
    you print it or not. Once you decide to print, however, the pixels per
    inch will be inverse to the size you print it. If you print it 4 x 5,
    you will have more ppi than if you print it 8 x 10 (twice as many, in
    Don Stauffer, Jan 18, 2004
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