Digital Images for the Web

Pixels Are It

For a digital photo or a scanned image to be described, the computer breaks the picture up into tiny pieces called pixels. Each pixel holds a small amount of information (data), and when they are put together on a grid, an image appears.

PPI

The image resolution is usually expressed in pixels per inch (ppi). The more pixels that are in an inch, the better the detail of your image. Naturally, you would think then that the highest resolution would be the answer to great images. This is a common mistake among novices. What resolution you create or scan an image at depends on variables, such as what type of paper your final print will be on as well as the resolution of your printer. All printers have only so much resolution. If you are creating or scanning for screen presentation (i.e. the Web or multimedia usage), you will only work at your monitor's resolution that is usually 72 ppi to 96 ppi.

As mentioned, most monitors have a resolution of 72 ppi to 96 ppi. Your monitor does not affect the resolution of an image; it only affects how the image appears on screen. If your monitor only shows 72 pixels in an inch and you open a 300 ppi file, it will appear bigger, because again, your screen can only show you 72 pixels in an inch.

DPI

The most commonly used resolution is dots per inch (DPI). The correct place for this term to be used is in reference to the output resolution of an imagesetter. The typical range for an imagesetter is 1200DPI to 5000 DPI.

SPI

A sample per inch (SPI) is generally the way that scanners have their resolution measured. Scanners commonly have resolutions between 300 and 4000 SPI.

LPI

The other area where we need to discuss frequency is on the output of printed pages. Here, we refer to the resolution of the halftone screen, which is measured in lines per inch (LPI). Low quality printing will have line screens in the area of 65 LPI (like a newspaper). Most magazines printed today have resolutions of 150 LPI.

Rule of Thumb

There is one rule of thumb that is particularly useful when trying to determine how much resolution is necessary for an image. Try to shoot for an image resolution that is 1.5-2 times the line screen resolution of the finished size image. The 1.5-2