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Image Scanning - Confused About DPI?

One of the first settings you need to choose when scanning an image is the DPI (Dots Per Inch). This is a very confusing parameter and is often set incorrectly. Scanning an image with an incorrect DPI setting can result in a blurry or slow-loading picture.

The first place you encounter DPI is your computer screen. Let's assume that you have a 19 inch monitor. The 19 inch dimension is the "diagonal" measurement of the front of the CRT (Cathode Ray Tube). Because the CRT needs trim around it, the actual visible screen dimension is smaller than 19 inches.

1024 x 768 Computer Screen

Note: LCDs (Liquid Crystal Displays) don't need trim so you get the full diagonal dimension as visible screen.

Your 19 inch CRT computer screen ends up being about 15 inches in width.

The image on the screen actually consists of an array of dots called "pixels". If you set your screen to 1024 x 768 resolution, your screen has 768 rows of 1024 dots. Dividing 1024 by 15 results in your computer screen having 68 DPI. To scan an image to appear the same size on the screen as the original picture, set the scanner to 68 DPI.

There are many different size computer screens, all set to various resolutions from 640 x 480 to 1280 x 1024 and higher. Using the above method, you can calculate the DPI for your specific computer screen. Your image will only appear "actual size" on screens that are the same size and set to the same resolution as yours.

Note: New technologies like "style sheets" and "XML" are designed to let you display an image with a specified size on any computer screen, but this has not yet been perfected.

Size on computer screen

Size on printer

Let's assume that you scanned a three inch square picture at 68 DPI so that it will display in actual size on your computer screen. Then you send it to your printer. Surprise! Your printer prints a 3/4 inch square picture. That's because your printer is is 300 DPI.

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