DPI

Discussion in 'Photography Beginners' Forum' started by LiveStrong2009, Jun 14, 2010.

  1. LiveStrong2009

    LiveStrong2009 TPF Noob!

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    I am rather confused about one thing, hopefully somebody has a simple answer for me.

    I am taking pictures for somebody, they requested pics that are at least 300DPI. I looked at the specs of all my pics and find all of them to be 72DPI. Why are they 72 DPI? How do I change this?

    I tried changing from a midrange quality to RAW, but they all remain 72DPI.
    I am shooting with a Rebel T1i.

    Thanks!
    -LiveStrong
     
  2. katy625

    katy625 TPF Noob!

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    I change mine in photoshop.....do you have an editing program? When you open a pic in ps (i have ps elements 7) you can go to Image - Resize - Image Size - then change the resolution to 300. U need this resolution when printing pic especially if you are printing pics larger than 8X10 because obviously a higher resolution will result in a clearer pic.
     
  3. Boomn4x4

    Boomn4x4 TPF Noob!

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    Your DPI dosen't have much to do with the image itself... it has more to do with printing. In fact, the DPI that you are looking at (assumingly from the EXIF data) is pretty much just made up... and 72 DPI happens to be the default DPI for displaying images on a monitor. The reality is, an image does not have a DPI until you actually print it. In other words the DPI only exists in the presentation, not in the image itself.

    Any image that comes off of your camera can be printed at 300 DPI. The actual size of the print at which can be printed at 300 DPI will change depending on the resolution your camera takes it at... but it can still be printed at 300 DPI. To find the print size at which you can print at 300 dpt, you simply need to divide the resolution by 300.

    For example, an 8MP camera takes pictures at a resoltuion of 3264 x 2448. Divide 3264 / 300 and Divide 2448 / 300 and you get 10.8" x 8.2". So you would be able to give them pictures that are 10.8" x 8.2" in 300 DPI
     
  4. KmH

    KmH Helping photographers learn to fish Supporting Member

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    DPI = Dots per inch. Digital photographs are PPI = Pixels Per Inch

    Dots are round. Pixels are square or rectangular.

    DPI is a value applicable to some types of printer.

    It takes up to 8 dots to print a single pixel.

    In other words, DPI is not the same thing as PPI.

    Boomn4x4 is correct that PPI is a meanless value until an image is going to be printed.

    Otherwise, for screen display, only the pixel dimensions have meaning.

    In this thread I have posted 3 versions of the same image, all at the same pixel dimensions but at 300 ppi, 72 ppi, and 10 ppi: http://www.thephotoforum.com/forum/general-shop-talk/206419-need-advise-issue-website-images.html - they all look the same because on screen PPI is meaningless.

    So for the OP if the client wants to print the image, you can change the resolution from 72 ppi to 300 ppi with image editing software.

    If the client only wants the image for screen use, do the above to keep them happy.
     
  5. Dwig

    Dwig TPF Noob!

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    Digital image don't actually have inches so, while they have pixels, they can't have pixels per inch. As a result, what you are seeing is not an actual property of the image. It is, instead, just a little note attached to the file that tells software that "the creator of this image thinks it should be printed at X ppi making the print Y inches by Z inches". Some software ignores it and some respect it.

    Also, many files, particularly those created by digital cameras, lack the note altogether. When software that supports PPI reads files lacking a PPI setting it must insert some default value. The most common such default is 72ppi. Adobe Camera RAW allows its inserted value to be set in preferences with most versions using 240ppi as their initial out-of-the-box default. When you see 72ppi you may well be seeing an image where the application is saying "hey, this file didn't specify a PPI so I'm assuming 72ppi".

    Regardless of how the PPI gets set, the PPI value is meaningless unless either the pixel dimensions or the inch dimensions are also mentioned. Saying you want 300ppi files is saying nothing unless its done in a context where it is safely assumed that the known cameras files have the desired pixel dimensions (number of pixels in each direction).

    To change the PPI in an image editing app without actually changing the image pixels (resampling) you simply turn off resampling. In PS, its a check box in the Image Size dialog (uncheck "resampling", type in the desired PPI, and click OK).
     

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