Image resolution in pixels vs DPI

When working with the print shop to create a poster out of a 3174 x 2153 pixel digital photo, I managed to get completely confused about DPI (dots per inch). I read a blog post called The Myth of DPI to educate myself, but that seemed to muddy the water even more for me. I must have been having a bad week because now the relation between pixel resolution and DPI is crystal clear:

pixels = inches * dpi

I wanted to print a poster that was 36″ wide, meaning that the DPI of the printout would be 88 (dpi = 3174 / 36 = 88).

Obviously, the more ink dots per inch you have, the crisper the image will look on a printed page. The printer recommended that I have 300 dpi to retain the quality of the image. What they didn’t make clear to me was that in order to have a 36″ wide poster at 300 dpi, I would need an image that was 10800 x 7200 pixels.

Adding to my confusion is that DPI is apparently part of the JPEG metadata. This means that you can change the DPI to whatever value you want. The key point is that raising the DPI reduces the printable image size, and lowering the DPI increases the printable image size.

All of this is spelled out in the The Myth of DPI, but for whatever reason I wasn’t getting it. Once I finally understood, I decided to bite the bullet and print the size I wanted, meaning that I would have to deal with 88 dpi. I think the resulting poster looks great. I mean, you wouldn’t put it up in an Ansel Adams gallery, but I don’t see any pixelation and it will be viewed mostly from afar. Time to frame this puppy and call this project done!

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