Quote:
Originally Posted by u238110
Does anyone have a sense of what DPI to use for ebook images? It seems to me like many scans are artificially large in size. For example, 300 DPI is popular, and I don't know if that's always necessary. If the book is brand new, you can include large images in the ebook and then people will have the extra benefit of viewing the images in all their glory on a computer, but if you're just scanning an old book that has images that have been greatly downscaled already ...
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There is a lot of confusion about DPI & PPI out there. Here's an outline:
DPI = Dots Per Inch. This refers only to the resolution of a "marking device" (read: that part of a printer that actually marks the paper). Electronic files dont't have "dots" and thus don't have DPI.
PPI = Pixels Per Inch. This is the proper term to use when referring to digital images despite Photoshop's incorrect usage of DPI. Electronic files don't have inches so this value doesn't actually specify the resolution of the file. Only the image's size in pixels does.
When scanning printed images you generally need to scan at around 300 DPI to retain all of the detail visible to the average eye. If the images are halftone images (printed using dots of varying sizes) you may encounter moire problems. These occur when the halftone frequency and the scanning frequency interfere with each other. When this happens simply rescan at a different DPI.
The PPI set in a digital image is merely a note to be read by the application that opens the image. If that application has the concept of a virtual inch (e.g. word processors, page layout programs, ..., anything that thinks in terms if a printed page size) then that application can use the PPI to decide how large, relative to its virtual page, to scale the image. True ebooks (read: ePub, MOBI, ..., but not PDF) don't have virtual page sizes so as a rule ebook reading software will totally ignore any PPI set in the file. They will usually display the image 1 pixel in the file to 1 pixel on the screen. Most will override this and scale the image down when the image exceeds the size of the screen.