What does DPI mean and how does it affect images displayed on screen versus printed?
DPI, or Dots Per Inch relates to the dot density when printing.
To help better understand the relationship of DPI to pixel dimensions, take an 800x600 pixel image for example:
DPI stands for
It's used to descibe the output resolution of printers, and it's also often used instead of the PPI (Pixels Per Inch) unit, which is more appropriate for describing the resolution of computer screens, scanners and image files.
So, a printer might have a resolution of 2400 DPI, but that doesn't mean that you can print an image at 2400 PPI. The printer uses dots of only a few colors (usually cyan, magenta, yellow and black) which are combined in patterns to produce the entire spectrum in different intensities. A reasonable resolution for an image to be printed is 200 - 500 PPI.
If you for example have an image with the specified resolution 300 PPI, it will need 300 pixels for each inch when printed. To print as 4" x 6" the image needs to have the dimensions 1200 x 1800 pixels.
If you print this image using a printer setting of 1200 DPI or 2400 DPI, it will still be the same size, but the latter will have better quality because the printer fits more ink dots in the same area.
If you want to publish an image on the web, the PPI setting in the file has no relevance. The image is displayed with one image pixel per screen pixel. The resolution of a screen is usually around 100 PPI, so that's what people tend to specify when creating images for the web, eventhough the value is ignored by the browsers.