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.
Types of resolution
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), whereas images in ImageReady are fixed at 72 ppi.
The number of pixels per unit of length on a monitor is the monitor resolution, usually measured in dots per inch (dpi). Image pixels are translated directly into monitor pixels. In Photoshop, if the image resolution is higher than the monitor resolution, the image appears larger on-screen than its specified print dimensions. For example, when you display a 1-inch-by-1-inch, 144-ppi image on a 72-dpi monitor, the image fills a 2-inch-by-2-inch area of the screen. ImageReady images have a consistent image resolution of 72 ppi and display at the monitor resolution.
The number of ink dots per inch produced by an imagesetter 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.
Resolution for this lesson
To determine the image resolution for the photograph in this lesson, we followed the computer graphics rule of thumb for color or grayscale images intended for print on large commercial printers: Scan at a resolution 1.5 to 2 times the screen frequency used by the printer. Because the magazine in which the image will be printed uses a screen frequency of 133 lpi, the image was scanned at 200 ppi (133 x 1.5).
For complete information on resolution and image size, see Adobe Photoshop 7.0 online Help.