preparing iPhone 12 pro photos for print using photoshop

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@nautilusnine wrote:

My problem is when I import my photos to photoshop, the dpi shows up as 72.  I know I can change it to 300, but will I truly retain photo quality that way.

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The photos have a specific width and height in pixels, 4032 by 3024 pixels for the standard iPhone 12. There is no inherent ppi value at the sensor, so the camera might not embed one in the images. If an image has no ppi value, Photoshop and Bridge assume 72 ppi. This might be where the 72 ppi value comes from — it might have been a number the software threw in there because there wasn’t a number in there.

Now, PPI does matter in printing…a lot. But where many people trip up is that ppi means nothing on its own. It only means something in the context of the physical size of the image. PPI is pixels per inch; someone mentions 72 or 300 ppi and that is supposed to mean something, but it doesn’t yet, because you have only one of the two numbers you need: The pixels. The other number you need is the inches. How many inches long do you want to print that? When you can state both pixels and inches, now ppi means something. For example:

If a standard iPhone 12 image is 4032 by 3024 pixels, and we say 72 ppi, what does that mean? If we divide 4032 by 72, we get 56. That means the 4032 by 3024 pixel image is 72 ppi if you print it 56 inches long. But you probably don't want to print it that long, so 72 ppi is not a useful value. How about you want a print that’s 10 by 8 inches? OK, let’s divide 4032 pixels by 10 inches…we get 403.2 pixels per inch. By stating our intended physical print size, 10 inches, now we know the image will be over 403 ppi for a print 10 inches long. That is great, because the common guideline is to aim for 300 ppi for a print.

How big can the iPhone 12 image print at 300 ppi? 4032/300=13.44. If you want to make prints at no less than 300 ppi, the iPhone 12 images can be up to 13.44 inches long.

But the 300 ppi guideline is just a guideline, and it is for a close viewing distance, on excellent paper, and assuming the image is properly sharp (no motion blur, misfocusing, etc.). If the image will be viewed from a distance like a poster or billboard, or it is a little blurry because your hand shook while taking the picture, or it wasn’t focused properly, or you’re going to print on a cheap matte paper, then a lower ppi value such as 250 or 200 ppi might look just as good as 300 ppi.

So in short, don't get too hung up on the exact ppi value you see at first in software. The most important thing is that you understand what ppi value is actually going to be useful given the image and type of printing and viewing (300? 200? 150?), and understand that the final ppi value is determined by how many pixels you are spreading out across how many inches. In other words, how much are you scaling the image at print time. Larger print sizes result in a lower ppi value because of the math above. If it works out to 300 ppi at 13.44 inches, if you print it 30 inches long it will be about 134 ppi. Spreading the same number of pixels across a larger print size in inches.

If you want to play with those print size vs ppi calculations, you can do it in the Image > Image Size dialog box in Photoshop, with the Resample option disabled.