Because printers and screen devices think about pixels differently, how many pixels you need depends on how you plan to use your digital picture. So, the answer to the pixel-number question is, “It depends.” Pictures’ pixel needs vary:
Pixels for screen use: If you want to use your picture on a Web page, a multimedia presentation, or for some other on-screen use, you need very few pixels. Just match the pixel dimensions of the picture to the amount of the screen you want the image to fill. In most cases, 640 x 480 pixels is more than enough, and for many projects, you need half that many pixels or even fewer.
Pixels for prints: If you plan to print your photo and want the best picture quality, you need enough pixels to set the output resolution in the neighborhood of 200 to 300 ppi. This number varies, depending on the printer. Sometimes, you can get by with fewer pixels; some printers ask for 360 pixels.
To determine how many pixels you need, just multiply the print size by the resolution you want. For example, if you want to create a 4-x-6-inch print at 300 ppi, you need at least 1200 x 1800 pixels. This figure shows some of the most common print sizes and the number of pixels you need for good prints.
