We regularly work with printers, or work with people who work with printers, and there’s an incredible amount of confusion about pixels, dots per inch and what the print shop really wants. In trying to explain it all to somebody, I stumbled upon two decent articles that can do the explaining for me.
The first is called The Myth of DPI, and covers the basics and the common mistake:
The client already has a beautiful digital photo with pixel dimensions of 2048 x 1536. The client notices that the photo editing software is showing that the photo is set to 72 dpi. So, following orders, the client types in 300 to reset the dpi to 300. In doing so the image is resampled and is enlarged over 4 times to pixel dimensions of 8533 x 6400. The client sends this enlarged 300 dpi photo. The print shop/graphics designer/magazine reject it (too grainy, too colour blotched). The client is crushed.
And here’s the second, on what the print shop really wants:
What they are really asking for is a photo that will print at a certain paper dimension in inches at 300 pixels per inch (PPI). If you remember from the Myth of DPI, the term DPI is a holdover from when this setting in a digital photo would set the paper output quality (resolution) of a printed image (number of printer dots per inch). This is no longer the case, but people still confuse DPI with PPI.
Go forth and print your materials with confidence, knowing they won’t come back all jaggy and blotchy.