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EthanTown

Q: Photo Resolution after Touch Up

Does using the touch up edit function in Photos change the image resolution when I export the photo with changes?

 

I scanned slides at 600 dpi, then imported into Photos. When I export, the resolution is now 72 dpi (according to Preview).

Posted on Jan 13, 2016 6:25 AM

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Q: Photo Resolution after Touch Up

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  • by Terence Devlin,Solvedanswer

    Terence Devlin Terence Devlin Jan 13, 2016 5:32 PM in response to EthanTown
    Level 10 (139,532 points)
    iLife
    Jan 13, 2016 5:32 PM in response to EthanTown

    Short answer: It means nothing. The dpi is set when you decide what size you're printing at.

     

    Long Answer: Dpi means nothing in the digital world of your computer. There are no "inches" to have "dots per..." Size is measured in pixels. That's the same on your camera. It doesn't take 10 x 8 or 6 x 4 shots. It takes shots measured in megapixels. For instance 4,000 x 3,000 is a 12 megapixel camera.

     

    Using that example, that shot from that camera has 12 million pixels. So that's how many "Dots" there are. To decide the ratio of dots per inch, you now need to decide the "inches" part. And that's printing. Print at 10 x 8 and the dpi will be 4,000/10 or about 400 dpi. At 6 x 4 then it's 4,000/6 or 660 dpi. Work the other way: Print at 300 dpi and the resulting image will be about 13 inches on the longer side.

     

    So, your photo as a fixed number of pixels. Changing the dimensions of the print will vary the dpi, changing the dpi will vary the dimensions of the print.

     

    For more see http://www.rideau-info.com/photos/mythdpi.html

     

    Regards

     

    TD

  • by clodo9,Helpful

    clodo9 clodo9 Jan 13, 2016 5:32 PM in response to EthanTown
    Level 2 (194 points)
    Jan 13, 2016 5:32 PM in response to EthanTown

    Yes, Photos kinda "default-resets" the dpi of an edited pic to 72 dpi ( like Preview has been doing for years ). But don't worry, the quality of the pic is NOT affected.

    See : 

    in the excellent " All about digital photos ", by Ken W. Watson,

    " The Myth of DPI " at 

    http://www.rideau-info.com/photos/index.html

  • by EthanTown,

    EthanTown EthanTown Jan 13, 2016 5:33 PM in response to Terence Devlin
    Level 1 (0 points)
    Jan 13, 2016 5:33 PM in response to Terence Devlin

    THANK YOU so much for this thoughtful, detailed reply. This makes sense.