Image Resolution
What is meant by "resolution"?
The term "resolution" as applied to images, can mean different things, depending on whether you're talking about print or digital media. For print media, the number of dots per inch (DPI, dpi) makes a tremendous difference. Compare an image that was printed on a dot matrix printer at 72 dpi with one printed on a laser printer at 300 dpi.
Pixels per inch (PPI)
For scanned digital images, what determines the quality of the image is the number of pixels per inch (PPI, ppi) that the image is scanned at. A 4 x 6 inch image scanned at 72 ppi will be 288 x 432 pixels. The same image scanned at 400 ppi will be 1600 x 2400 pixels. It is important to note here that any good image manipulation program can change the number of pixels per inch of a digital image without changing the number of pixels in the document. Depending on the amount of detail in the printed image, an image scanned at 600 pixels per inch (ppi) will have more detail than an image scanned at 300 ppi.
You will find the terms dpi, DPI, ppi and PPI used interchangeably at DP.
High-Resolution
DP commonly uses "high-resolution" to refer to illustration images that have been scanned at 400+ ppi, and "low-resolution" to refer to images that have been scanned at fewer ppi, though the quality and size of the original should be taken into account when determining what the appropriate scanner setting is. A large image with no fine detail can be adequately scanned at 300 ppi, but a small image with a lot of fine detail may need to be scanned at 600 ppi to adequately preserve the detail.
Examples
The screenshots below show two perspectives on the image size metadata for the same image. The original scan is from TIA (unique identifier: childsgardenofve06stev). On the home page for that book, the scan resolution is reported as 400 ppi. Notice that reported image size (inflated by the program) and dimensions are the same.
Checking image resolution
When you check image resolution in an image processing program, you should also be aware of the overall size of the image in pixels, and what it thinks the dimension of the scan is relative to what is likely to be the size of the book (a book that is 40 inches wide by 60 inches high?!)
