Resolution and image size
The first step in retouching a photograph in Photoshop is to make sure that the image is the correct resolution. The term resolution refers to the number of small squares known as pixels that describe an image and establish its detail. Resolution is determined by pixel dimensions, or the number of pixels along the width and height of an image.
Pixels in a photographic image
In computer graphics, there are different types of resolution:
The number of pixels per unit of length in an image is called the image resolution, usually measured in pixels per inch (ppi). An image with a high resolution has more pixels (and therefore a larger file size) than an image of the same dimensions with a low resolution. Images in Photoshop can vary from high resolution (300 ppi or higher) to low resolution (72 ppi or 96 ppi).
The number of pixels per unit of length on a monitor is the monitor resolution, also usually measured in pixels per inch (ppi). Image pixels are translated directly into monitor pixels. In Photoshop, if the image resolution is higher than the monitor resolution, the image appears larger onscreen than its specified print dimensions. For example, when you display a 1-x-1-inch, 144-ppi image on a 72-ppi monitor, the image fills a 2-x-2-inch area of the screen.
4 x 6 inches at 72 ppi; file size 364.5K
100% onscreen view
4 x 6 inches at 200 ppi; file size 2.75 MB
100% onscreen view
The number of ink dots per inch (dpi) produced by a platesetter or laser printer is the printer, or output, resolution. Of course, higher-resolution printers combined with higher-resolution images generally produce the best quality. The appropriate resolution for a printed image is determined both by the printer resolution and by the screen frequency, or lines per inch (lpi), of the halftone screens used to reproduce images.
Keep in mind that the higher the image resolution, the larger the file size and the longer the file takes to download from the Web.
For more information on resolution and image size, see Adobe Photoshop Help.