DPI stands for dots per inch. PPI stands for pixels per inch. Both these words are describing the resolution of an image. If you have an inch, the DPI or PPI is how many dots or pixels fit within that space. While PPI is used to talk about digital pieces and DPI to reference printing, both can be used interchangeably in today’s world. Both reference the number of pixels in a piece of work.
The lower the DPI, the fewer pixels you have in a certain amount of space. The higher the DPI, the more pixels you have. You are expected to use 72 DPI for a digital screen, which makes resolution crystal clear on almost any screen. The digital screens for the Mac retina displays, 4k monitors, and e-readers for the new Kindles Paperwhites are 300 or 400 DPI.
For print, you never want to go below 300 DPI because your collateral will become blurry. Besides, there’s usually never a good reason to print above 450 DPI as the increased resolution follows a law of diminishing returns.
Any normal sized document with DPI above 450, aside from huge billboard or building signage, won’t make the images or elements of a page any crisper, and it’ll just eat up your file space. In InDesign, we set our PDFs to trim down any files over 450 DPI to save on file space.
InDesign also has a unique tool called effective DPI. As an example, an image is 1,000 pixels x 1,000 pixels at 100 DPI by shrinking that down to 1/3 of the original image, that would bring it down to 333 pixels wide. That image will go from 100 DPI to 300 DPI. Basically, making the image smaller increases its DPI.
Let’s say you are creating a newsletter, and on the back of it, there is a page with all the board of directors for the non-profit that the newsletter is a part of. Each of the photos is ultra-high-resolution portrait photos with huge files, around 20 MB each.
However, the board of directors was only postage stamp sized image on the back. While the images started at 300 DPI, sometimes they may become 1,000 to 2,000 DPI when they are made that much smaller. You really don’t need the resolution that high, so what we do is set InDesign to automatically drop our resolution down to 300 DPI to save on file storage space.