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I am currently working on a print media project that requires a 300 DPI minimum for all images. I thought I had to discard a bunch of photos that were 180 DPI at their default resolution. However, I recently realized that you could set the Pixels/Inch in Adobe PS under Image->Image Size. So I tried seeing if I could set the image too 300 DPI, which it did, but then it got larger, which befuddled me. If there is an upper bound on an image's quality by virtue of the camera with which it was shot, how is it that an image that has a default of 180 DPI at a certain resolution can magically get greater density of visual information AND get bigger?

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You need to uncheck the "Resample" checkbox before you change the ppi number. That leaves the image unchanged. To be sure, ppi and number-of-pixels are related, but they are not the same. (PPI is the correct term for an image. DPI has to do with dots on a piece of paper, and may not have any relationship with the PPI of the image being printed.)

An image has a certain resolution in pixels. Its metadata contains a value of pixels-per-inch that defines the size of the printed image.

That said, you don't need to change the ppi value in Photoshop at all, because it will have no effect on the printed result. When the designer places an image into a layout in InDesign or QuarkXpress, he or she will scale it to fit the needs of the layout. That placed image will have an effective ppi that is based only on the number of pixels it contains and the dimensions it has in the layout. The ppi number from Photoshop is noted, but has no relevance in the final output.

If your image must be 4 inches square on the page, it must be 1200 pixels square with no cropping, larger if it will be cropped. The ppi value reported by Photoshop might be 100 ppi, in which case Photoshop will also report that the document is 12 inches square. But when it's placed in the layout at 4 inches square it's a 300 ppi image, regardless of what Photoshop "thinks."

What you need to know is "how big," not "what ppi." Find out what size in inches the largest images will be in the layout. Multiply that number by 300 to get the smallest image size that will meet the spec.

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@Alan_Gilbertson thank you so much! Very edifying. Is PPI to DPI a one to one ratio? I have a spread in inDesign that has frames that are 4.84 inches x 8.78 inches. So, given your formula, it follows that I need a photo with a resolution of 1452 pixels x 2634 pixels. Would this not just guarantee PPI of 300 but also DPI of 300? Or does it depend on the printer... –  Thalatta May 16 at 22:38
    
Your math is good. :) PPI has to do with images ("pixel" is a contraction of "Picture Element). DPI has to do with ink, toner or physical displays. If your desktop printer prints at 1200 dpi, it's going to print at 1200 dpi no matter what ppi your image has. Unless you're planning to spray ink on your monitor (in which case you have much bigger problems than anyone here can help you with), you don't need to sweat dpi. –  Alan Gilbertson May 16 at 23:30

Uncheck Resample when increasing the PPI. You'll see the image get smaller (which it's supposed to).

With Resample checked, you are increasing the number of pixels in the image, but not increasing their density (ppi) therefore the image gets larger to compensate for the additional pixels. By unchecking Resample you are increasing the pixel density and this will traditionally result in a smaller (width x height) image.

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