DPI and PPI seem to confuse a surprising number of people. But what really surprises me is how the topic comes up... it seems like I'll usually hear someone ask this question because a "publisher" told a "photographer" that they need an image at, say... 300 dpi.
Immediately two things go through my head which is (1) why is a publisher even asking this question and (2) do they not know Photoshop well enough to realize that they, the publisher, are in control of this setting and they can use Photoshop to set any DPI value they want?
Both DPI and PPI only makes sense when you are trying to display the final product... either "print" the image in a magazine, newspaper (what's a newspaper?), etc. or even to print it for purposes of mounting and displaying it on a wall. PPI assumes it'll be digitally displayed (on a screen).
A publisher may not make the DPI statement in the right context, but a printer would. DPI will make sense in a finished printed piece. They will typically request all images at 300dpi minimum, and mean it. And no, they are not in control of this setting if they get a finished layout from Quark or InDesign, or a PDF from either. They get what they get, and will not attempt to change anything in the document...its like a rule of the printing industry, "we only print what you send us." BTW, that means we try to predict what the press will do to our greyscale too, and pre-compensate. (see Dot Gain)
Offset printing is very different from digital inkjet or laser printing, or fine art printing. Because offset printing uses a halftone screen at 45 degrees, with a variable dot size, hitting that process with an image below a certain finished dpi creates issues. The rule (with the printing company we use) is 300dpi, finished size. We work in QuarkXpress, our printer uses a digital plate maker. So, if we start with a 3000px wide image, but it has to fit in a 4" wide box on the page, we resize it down 300dpi. There are two reasons, sort of working against each other. First is, at at the finished screen pitch the printer uses, 300dpi works and causes no issues. Below that we can get moire patterns, and it's difficult to predict in the page-layout stage. Going higher than 300 causes no problems for the printer, but it does cause problems for us. Depending on Quark to handle all the resizing results in clumsy and huge Quark documents. While the days of Quark not resizing photos well are over (resizing in Photoshop was the rule back then, Quark made a mess of things), we still pre-size in PS before import because the photo and files are just smaller and faster to handle. There are also a few steps of pre-press photo processing to go through too, so that happens at that point as well.
We sometimes get very low resolution .jpg files that we have to print. They actually get resized up to end up at 300dpi too, again, avoiding interference patterns with the printer's screen. The photo resolution is not improved, of course, but at least one problem is avoided. And we do line-art at 1200dpi, bitmaps, which is absolutely necessary for crisp, smooth line art (no screen), but completely unnecessary for photos.
