This post is about something that bothers me a lot. Especially, because it originates from a place where you think they should know better. It's about Dots-Per-Inch (DPI) and JPEG (the popular digital image/photo format).
It all starts, when I read the requirements of certain online photo contests. The criteria for entering the contest contain the following: The photo entering the contest must be in JPEG with maximum quality (least compression), AND 300 DPI.
And that is where things start to itch. 300DPI... That doesn't mean anything inregard to a digital photo, because 300 DPI is something used in print, and NOT in digital media (photos). A digital photo consists of pixels horizontally, vertically, and there the number of bits per pixel containing color information. Sure, DPI (or Pixels-Per-Inch / PPI) can be a value in the EXIF information of a photo, but it doesn't say anything about the quality of that photo. And still 'they' keep using it in their contest requirements.
It would be better if they stated that the photo should be at least 10cm * 15cm @ 300DPI (or PPI), which means that the JPEG should be at least ((10/2.54)*300) * ((15/2.54)*300) = 1180 * 1770 pixels in size (note: 2.54 cm is 1 inch). Or just mention the total number of pixels which will also include square photos, and/or panoramic sized photos.
The following should illustrate that maximum JPEG quality and a certain DPI value doesn't say anything about quality.
First; You can alter the image size in Photoshop in several ways, one of them being the DPI. If you change the DPI, the other settings are changed accordingly (might depend a bit on the settings used).
The settings below, both represent an image of 2954 pixels * 2954 pixels. Only difference is the document size. If you would send these images in 'professional' format (e.g. TIFF, or PDF) to a printing service, you should get a print 100 * 100 centimeters and a print of 25 * 25 centimeters. The difference in size is determined by dots per inch the printer is using to transfer the digital photo to another media (e.g. paper).
If you should save these two images as JPEG to disk (with the same JPEG quality), you get 2 identical files in size and quality.
Below are the actual images used. On the left the 75DPI version, and on the right the 300DPI version.
I can manipulate the same image which has a much larger DPI (or PPI), but looks much worse than the 75DPI image. But the 75DPI image can't enter the photo contest, because it fails to meet the requirements.
The following photo is the same photo used earlier, but I altered the DPI (which in turn alters the actual size by default in Photoshop). The resulting image is 400DPI. It was saved with the maximum quality possible, but results in a photo of a mere 630 * 630 pixels. This can basically only be used as a profile photo on Twitter or Facebook. Even a relatively small print will result in a bad print.
The following shows the difference in quality. The one the left is the 400DPI photo with maximum JPEG quality, and on the right the 75DPI photo. Both with the same JPEG compression. The left photo can enter the photo contest, since it meets all the requirements, the one on the right is disqualified.