Pixels v Jpeg File Size v Print Size??

Discussion in 'Digital Photography' 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

    Tom Thackrey Guest

    Jpeg files are compressed so their file size has almost no relationship to
    anything except that if you know the uncompressed file size you can compare
    it to the compressed file size and determine the compression ratio.

    When people (who know what they're talking about) compare image quality in
    terms of file size, they are referring to the uncompressed file size. In
    general, the uncompressed file size will be the dimension in pixels times 3
    because it takes 3 bytes to hold the color information for one pixel (in
    8bit/color RGB). A 3000x2000 pixel image will be an 18 MB file

    Pixel dimensions are the issue. Depending on the printer, somewhere between
    about 180 and 350 pixels per inch (PPI) will produce the best print that
    printer is capable of printing. So, if you are printing a 4x6 inch image on
    a printer that does its best work at 300ppi, you will need at least a
    1200x1600 pixel image (4*300 x 6*300). That image would also be about a 6MB
    uncompressed file. Note that the DPI printed on the outside of the printer
    box is meaningless. Printers use multiple dots to make a pixel, so 1440 DPI
    has almost no relationship to the PPI needed for a good print on that
    Tom Thackrey, Jan 18, 2004
  4. 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
  5. PeterH

    Jim Townsend Guest


    Pixels and bytes are two different things. Pixels are picture elements
    that were captured by your camera. As a mater of fact that's where the
    term 'pixel' came from.. it's short for "picture element"

    Bytes are what computers use to represent data. The only thing they
    have in common is they use the same prefix, 'mega'. That designates
    a million. A million volts is a megavolt.. But there are no
    megapixels in a megavolt.

    Music files are made of bytes, video files are made of bytes. Program
    files are made of bytes. If you look on your hard drive, you'll see
    *all* your files are made of bytes. Almost all these files have
    megabytes but they have NO pixels.

    The number of bytes required to represent a pixel varies depending on
    the complexity of the pixel and whether it's color or not. For 8 bit
    color files, you need three bytes to represent the data in one pixel.
    So.. A 6MP file WILL equal 18 Megabytes.

    But your JPEG images are only 3 megabytes !! Yes.. JPEG is handy
    because it compresses the image data. This saves lots of space
    on your compact flash card and on your hard drive.

    Note that a 3 Megabyte JPEG still has 18 Megabytes worth of data..
    It's only compressed when you save it to disk.. When you open a
    JPEG file, it is uncompressed and exists as an 18MB file in your
    computer's memory... If you save your 3 Meg JPEG files as
    uncompressed TIFF files, you'll see they will be a full
    18 Megabytes..

    For printing.. I think this is the most misunderstood aspect of
    digital photography there is..

    Consider you have an image. It has pixels in two dimensions..
    That's width and height. Megapixels are derived by multiplying
    these two numbers together..

    In the case of your camera, the image is 3072 wide by 2048 high.
    If you multiply, 3072 x 2048 = 6.29 Million. Or 6.29 megapixels.

    If you print your image 14 inches wide, then you're spreading
    3072 pixels across 14 inches of paper. In doing this, you wind
    up with 219 pixels in every inch of the image.. Or.. 219 Pixels
    per inch. PPI.. Sometimes called DPI.. (Dots per inch).

    If you print the 3072 pixel wide image 6 inches wide.. Then you
    have 6 / 3072 = 512 Pixels spread out over each inch, or 512
    pixels per inch.

    That's *all* pixels per inch are.. The physical dimensions of
    the image printed on paper, divided by the pixels in your image.
    Pretty simple, but again.. It baffles many.

    The more pixels per inch you have, the better your printed image
    looks.. Makes sense.. There's more data.

    But you say your camera or software says the image is 180 pixels
    per inch (or some other value) !!!

    Yes.. You can tag whatever PPI or DPI value you want on an image
    file. This value has NO effect on the pixels or how they are
    arranged or the quality of the image.

    When you see a value of 'xxx' pixels per inch, you're looking at
    the *proposed* size the image will be *when* it is printed on PAPER..

    So if you see your editor showing your 3072 pixel wide image with a
    value of 72 pixels per inch, then that means if you don't change
    anything, it will print at 3072/72= 42 inches wide.. But again, if
    you don't want it that big, just change the pixels per inch. If
    you change it to 512 pixels per inch, then the image will print at
    6 inches wide..
    Jim Townsend, Jan 18, 2004
  6. 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
  7. 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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