11 Replies Latest reply: May 22, 2012 8:31 AM by Keith Barkley
LondonDave Level 1 Level 1 (0 points)

I've looked back at some previous discussions of the DPI setting in Aperture, and I've also read some online explanations of DPI vs PPI. Apparently software applications sometimes confuse the two, and I suspect that this is the case with Aperture but I haven't seen this definitely specified. Photography competitions that ask for high-res files often ask that you use 300 PPI (not DPI); but previous discussions of Aperture suggest that the DPI option (there isn't a PPI option) should be set to 300 for high-res exports. In other words, Aperture seems to refer to DPI when it should be referring to PPI. Is this correct?


MacBook Pro, Mac OS X (10.6.7)
  • 1. Re: DPI vs PPI - a definitive answer?
    LondonDave Level 1 Level 1 (0 points)

    By the way, I have just tried exporting a 300 DPI TIFF file from Aperture, and then reopening it in Pixelmator. In Pixelmator, the Info states:

     

    DPI: 300 pixels/inch

     

    Note - pixels, not dots!

  • 2. Re: DPI vs PPI - a definitive answer?
    Keith Barkley Level 5 Level 5 (5,260 points)

    There is no definitive answer, since people use the terms all over the place. It seems that most photography contests are the most definitively wrong.

     

    A computer image has only a size that is expressed in pixels.

     

    When you try to view the image, DPI comes into play. Monitors traditionally set 72 DPI, but are creeping higher and higher. In any case, the monitor typically scales the image to fit on the screen anyway.

     

    DPI is really meant for printing. Calculating the DPI will tell you the resolution or graininess of the image. Typically you shoot for 300 DPI for best quality, but for a large print - that is not viewed too closely - 220 DPI is also mentioned.

     

    The kicker is that DPI refers to a single dot of pure color, what you get with dye sublimation printing. Most inkjets use multiple dots to create a single pixel, up to 1200-2400 ink dots per per inch.

     

    I guess you are right that PPI should be used for pixels and DPI for ink dots, but that does not seem to be the way the term is used. Feel free to buck the trend, though! 8^)

  • 3. Re: DPI vs PPI - a definitive answer?
    Kirby Krieger Level 6 Level 6 (11,945 points)

    In practice, set the DPI setting in the Image Export dialog to whatever the competition asks for, and ignore it.  It is meaningless within Aperture, and almost certainly meaningless to anyone viewing the file.

     

    The rule of thumb is that you can make a sharp print when you have 300 pixels for every print inch.  So for competitions that require "hi-res" images, you will want to make sure that the dimensions you supply for a print accord with the pixel dimensions of your image file.  For example, an image file 450 x 600 pixels should be listed as (rule of thumb) 1.5 x 2 inches; and likewise, a print listed as 13 x 19 inches should have an image file about 3,900 x 5,700 pixels.

  • 4. Re: DPI vs PPI - a definitive answer?
    SierraDragon Level 4 Level 4 (2,695 points)

    Sorry, there is no dpi in images, except mistakenly. Digital image sizes are discussed in pixels. Where does Aperture talk about dpi?

     

    Like Kirby said, 300 ppi is a rule-of-thumb (with tiny exceptions) that generally defines the size of decent print at common sizes that one can expect from a certain number of pixels direct (not uprezzed) from the camera.

     

    However note that different output devices actually will optimize at different ppi. A magazine, for instance may prefer art submissions at 255 ppi because that suits their workflow and their output device. Many inkjets (Epson photo for one) on the very best images show better at 360 ppi than at some other number like 300 ppi. Or when uprezzing there may be a size like 240 ppi that prints better than 275 ppi.

     

    Large printing like posters goes to another spec that is defined by the distance vewed at; 40-100 ppi is common there to avoid unmanageable file sizes.

     

    Bottom line though is that original linear pixel counts are what matter. My experience with magazine work was that a well-shot digital image could be uprezzed 2x when necessary.

     

    -Allen

  • 5. Re: DPI vs PPI - a definitive answer?
    LondonDave Level 1 Level 1 (0 points)

    SierraDragon wrote:

     

    Sorry, there is no dpi in images, except mistakenly. Digital image sizes are discussed in pixels. Where does Aperture talk about dpi?

     

     

     

    DPI in Aperture is referenced at:

     

    File > Export > Version > Edit

  • 6. Re: DPI vs PPI - a definitive answer?
    SierraDragon Level 4 Level 4 (2,695 points)

    LondonDave wrote:

     

    SierraDragon wrote:

     

    Sorry, there is no dpi in images, except mistakenly. Digital image sizes are discussed in pixels. Where does Aperture talk about dpi?

     

     

     

    DPI in Aperture is referenced at:

     

    File > Export > Version > Edit

     

    Thanks! I finally found the DPI reference under Export Preset/Edit and "Size To."

     

    The DPI that Aperture is referencing there is not the image file, it is the output device (such as a display) and output devices are often defined in DPI. Specifically Aperture is giving us the option there to create a specific output preset (which will be in pixels) to size to a specific device. E.g. if we want our image to size at 4" x 6" on a 72 DPI display we just set to 4" x 6" and 72 DPI and Aperture will do the pixel counting for us.

     

    The image file, however, will always be in x pixels by y pixels dimensions.

     

    HTH

     

     

    -Allen

  • 7. Re: DPI vs PPI - a definitive answer?
    SierraDragon Level 4 Level 4 (2,695 points)

    LondonDave wrote:

    Photography competitions that ask for high-res files often ask that you use 300 PPI (not DPI); but previous discussions of Aperture suggest that the DPI option (there isn't a PPI option) should be set to 300 for high-res exports.

     

    PPI and DPI are often incorrectly used interchangeably but most of the time the improper usage does not hurt anything. However IMO we should ourselves endeavor to use the terms properly. Aperture uses the dpi term correctly because it is referring to an output device.

     

    Much of the time "photography competitions" are just stealing your image one way or another, so the image spec is just to get it into the form they want to harvest. <OK I am a cynic...>

     

    When the contest spec is making sense usually it is just specifying linear size (inches or centimeters) x ppi (like 300 ppi) to achieve consistent linear pixel dimensions, which are what really matter. So if they want to harvest what would be typical 8x10 print quality image files they specify 8" x 10" at 300 ppi: the important net result is that every contestant provides a (8x300) x (10x300) = 2400 pixels x 3000 pixels file.

     

    Or they could specify 24" x 30" at 100 ppi: net result  (24x100) x (30x100) = 2400 pixels x 3000 pixels file.

     

    Or maybe it is a size-righteous competition, in which size as viewed matters. After all, we all know some pix show well small while others demand large presentation. In that case they may only specify the ppi. The photog determines the presentation size. E.g. 300 ppi is specified and one wants to present at 4" x 4"  it would be a 1200 pixels by 1200 pixels image submission.

     

    -Allen

  • 8. Re: DPI vs PPI - a definitive answer?
    LondonDave Level 1 Level 1 (0 points)

    Thanks for your reply, and to everyone else who replied.

     

    The photo competition I was referring to was the Take-A-View Landscape Photographer of the Year, which is a pretty good competition. As it happens, I'd managed to slightly misremember or misinterpret what they'd said about PPI. Contrary to what I originally wrote, the competition did NOT ask for digital photos in 300 ppi. It actually says something along the lines of "Digital images must be of sufficiently high quality that they can be printed at 300 ppi".

  • 9. Re: DPI vs PPI - a definitive answer?
    Tony Gay Level 4 Level 4 (1,625 points)

    Hi Everyone, I believe the best visual explanation I have seen to explain pixels is the one on the EPI Centre website by David Henshall.

     

    http://www.epi-centre.com/basics/index.html

     

    This covers detail etc, Tone and Colour/Color.

     

    Seriously recommend visiting this and reviewing the information, suddenly it will all make sense.

     

    Tony

  • 10. Re: DPI vs PPI - a definitive answer?
    léonie Level 9 Level 9 (51,740 points)

    Great Tip, Tony, bookmarked!

     

    Cheers

    Léonie

  • 11. Re: DPI vs PPI - a definitive answer?
    Keith Barkley Level 5 Level 5 (5,260 points)

    Which still says nothing. You can print a 300 x 300 pixel image at 300 DPI. Of course, you get a 1" image, but it *is* 300 dpi!