Another resolution question (sorry!)

Discussion in 'Digital Photography' started by Jeff Taylor, Aug 24, 2004.

  1. Jeff Taylor

    Jeff Taylor Guest

    Well, I'm starting to get the hang of all this PPI, DPI, image size etc.
    thanks mainly to some excellent posts to this group.
    When I import images from my Fuji S7000 and open them in Photoshop, the
    default resolution is 72 ppi (which I gather is correct for viewing on a
    monitor).
    Tonight I imported some images from the wife's new Nikon Coolpix 3200
    and when I opened them in Photoshop, the default resolution is 300 ppi.

    So my question is, what determines the default resolution of imported
    images - do the camera manufacturers decide?
    All images were imported using Windows explorer as opposed to the
    software that came with each camera.

    Thanks Folks,
     
    Jeff Taylor, Aug 24, 2004
    #1
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  2. Jeff Taylor

    Jeremy Nixon Guest

    Yes. The "ppi" on a file from the camera is entirely arbitrary and
    totally meaningless.
     
    Jeremy Nixon, Aug 24, 2004
    #2
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  3. Jeff Taylor

    Jim Guest

    It certainly seems so. Of course, since all that matters is the pixel
    count, all you need to do is resize the image to whatever display ppi you
    choose. Just make sure that you tell the program not to interpolate.
    Jim
     
    Jim, Aug 24, 2004
    #3
  4. That depends on the monitor. Some are 72 ppi, some are 100 ppi
    and some are 144 ppi, etc.
    Yes. From a Canon G5 you would get 180 ppi, etc.

    Please also note that the only use Photoshop make of this number is
    to compute the "Document Size" which it display in the "Image Size"
    dialog. It is not used for anything else (for instance - it is /not/
    used by Photoshop to figure out what resolution to use for displaying
    the image at the screen - and it is not used by the the printer to
    figure out how big to print (there may be exceptions to this - but I
    don't know of any printer driver that pay any attention to the
    EXIF-embedded resolution)).

    In other words: You may safely disregard this figure - but /if/ you
    are not happy with whatever default your camera uses, you can change
    it to whatever you like in the Image->Image Size dialog in Photoshop.
    As long as you make sure that Resample Image is unticked, changing
    this figure will not change your image in any other way.
    I don't think that import route matters. The inital ppi is just
    a meaningless figure embedded by the camera firmware in the EXIF,
     
    Gisle Hannemyr, Aug 24, 2004
    #4
  5. Jeff Taylor

    Jeff Taylor Guest

    <snip> lots of useful info.

    Many thanks Gisle, Jeremy & Jim.
    Very helpful as usual - it all just gets clearer by the day :)
    What a great group this is!

    Best Wishes,
     
    Jeff Taylor, Aug 24, 2004
    #5
  6. Jeff Taylor

    Jim Townsend Guest


    The 'DPI' of an image is the 'Proposed print size'.

    DPI has *nothing* to do with the quality of the image file
    or quality of the pixels as they exists in memory.

    As a matter of fact.. The DPI setting consists of a
    couple of bytes at the beginning of the file. That's
    all that gets changed when you change DPI.

    The DPI in a file can be zero, but most cameras and image editing
    programs will add a default value. Some add 72, current Canon cameras
    add 180.. Others add 300.

    Note that dots Per Inch *cannot* exist without inches because dots
    per inch is nothing more than the desired print size (in inches)
    divided by the number of pixels.

    Setting a 1000 pixel wide image to print across 10 inches
    of paper will result in 1000 / 10 = 100 dots being spread
    across each of the 10 inches of paper. In other words, a
    1000 pixel wide image printed at 10 inches has 100 DPI.

    It's mathematically impossible for it to be anything else.

    If you change the DPI of your 1000 pixel image to 500,
    then the image will be 2 inches wide.. (1000 / 500 = 2)
    Again.. A 1000 pixel wide image printed on 2 inches
    of paper MUST be 500 DPI.

    If you check your software you'll see that there are *always* two
    things shown in the image 'size' window. These are: The DPI
    *AND* the size of the image print in inches. (Remember.. you
    can't have DPI without inches).

    If you grab a calculator, you'll always see the DPI is *NOTHING*
    more than the number of pixels divided by the *proposed* print
    size in inches.

    If you change the print size in inches, you'll see the DPI changes
    accordingly. (DPI = Pixels / inches)

    If you change the DPI, then the inches change.
    (Inches = pixels * DPI)

    Note there are ways to resample images when you resize. Resampling
    adds or subtracts pixels from your image file. If you change the
    image size, and resample, you're doing TWO separate things.

    The resampling process can sometimes fool people.

    Many people think changing a large image to 72 DPI will make it fit
    nicely in their monitors.. This is absolutely wrong. The only
    reason it fits is because IT WAS RESAMPLED.

    Changing an image to 72 dpi to fit the monitor is about as useless
    as changing an image to 600 DPI to match the printer :)

    A 200x200 pixel image will look exactly the same on any monitor
    at 10 dpi, 72,dpi 200 dpi or 1000 dpi...
     
    Jim Townsend, Aug 24, 2004
    #6
  7. I still think my proposed explanation is best. The ppi (pixels per inch is
    more meaningful than dots per inch) should be seen as an "IF' statement. If
    printed to screen at 72 ppi its this size....if printed to inkjet at 200 ppi
    its such a size....if printed offset at 300 ppi its such a size.
     
    Gene Palmiter, Aug 25, 2004
    #7
  8. The difference between DPI and PPI is fairly vague. I wouldn't say that
    any one is particularly more useful than the other. It's just
    terminology. Some people may be happy to call dots on paper pixels,
    others prefer to say dots, either way it's pixel dimensions that really
    matter as most other respondents to this thread have indicated.
     
    Eugene O'Brien, Aug 25, 2004
    #8
  9. Trying to determine the ppi of a monitor is not a particularly useful
    thing to do. It depends on viewable area which varies for all makes and
    models of monitors, plus you can easily resize the display area with the
    monitor adjustments, this will likely mean that if you actually got out
    a ruller you'd find the PPI could be something 76.665 ppi.

    It's best to just consider PPI fairly irrelevant for anything screen
    based. It's much less confusing to always think about pixel dimensions
    of images when working on screen.
     
    Eugene O'Brien, Aug 25, 2004
    #9
  10. Jeff Taylor

    JPS Guest

    In message <0YMWc.12873$>,
    There is usually no reason to set a PPI value for an image at all,
    unless you want the file to automatically print at a certain size with
    no user interaction. Just tell the program you are printing with what
    size you want the image to be on paper, when you are printing it. If
    the number of pixels would result in pixellation, then resample by 200%
    or 300% before printing, after saving the file (if desired), and say no
    to the changes when it is time to close the file, and you still have the
    original.
    --
     
    JPS, Aug 27, 2004
    #10
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