Why 72 DPI?

Discussion in 'Digital Discussion & Q&A' started by wxnut, Nov 7, 2010.

  1. wxnut
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    wxnut New Member

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    Why does my camera shoot at 72 DPI when I shoot on the Hi REs JPG setting and makes the file 60 inches by 40 inches, but when I shoot at RAW it opens up at 300 DPI and a file size of 14.5 inches by 9.7 inches?

    Doug
  2. gryphonslair99
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    gryphonslair99 New Member

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  3. KmH
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    KmH Helping photographers learn to fish

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    Your camera doesn't shoot dots-per-inch, it shoots pixels-per-inch.

    PPI is meaningless until an image is output for print. Until then, all that matters is the image pixel dimensions, which are the same regardless the display being used and the ppi setting.

    As an example a digital photo having pixel dimensions of 4000 pixel x 2000 pixel, would print at 40" x 20" at 100 ppi. (4000 pixels divided by 100 pixels per inch = 40 inches (the pixels cancel))

    The same 4000 pixel x 2000 pixel digital photo printed at 200 ppi would print 20" x 10". (4000 pixels divided by 200 pixels per inch = 20 inches (the pixels cancel))
  4. Garbz
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    Garbz New Member

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    To extend on what KmH has said, the camera doesn't shoot anything per inch at all. It just shoots a set number of pixels.

    The problem is that for legacy reasons JPEGs have a PPI tag that is required since originally the reason you'd save an image was usually to print it out at a specific size elsewhere. The JPEG specification requires PPI to be set, so what do you say it at? There's three numbers you can pull out of your arse:

    - 72PPI - The standard screen setting of a Mac in the 80s. Required in the 80s for accurate font rendering, and made completely irrelevant ... back in the 80s.
    - 96PPI - The standard screen setting of a PC in the 80s, ad infinitum.
    - 300PPI - some magical number very loosely based on viewing a 6x4" print at arms length by a person with 20/20 vision and not being able to resolve the individual pixels.

    Two things come out of this:
    1. It only matters if you're designing fonts for display of computers made in the 1980s.
    2. It only matters when you go to print.


    Ignore it, it doesn't matter what you're going to do it will only ever be a relevant number for you if you're preparing a file to send to a printing company after post production, and even then ONLY if the printing company specifically state "we need a file at xxxPPI" or "we use the PPI to determine what size print you want". A large number of companies will flat out ignore the PPI setting and ask you what physical size you want the prints too.

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