Here is a handy way to calculate the DPI scan resolution needed to optimize your output image. Just select the output desired (inkjet or LightJet), enter the appropriate dimensions (in either feet or inches), and click "Enter" The resulting DPI can then be used to establish your scanner's setting.
Most people quickly find out that a scan which will fit on a single diskette doesn't look very good when enlarged to four feet. Scanners produce "digital images" that are made up of a square pattern of points (pixels). The colors recorded at each of these points will vary depending upon the type of scan to be produced. For RGB scans, 3 bytes of data is required ( one for each color channel ). This produces the following equation for determining the size of a scan. Popular usage refers to pixels-per-inch as dots-per-inch, or DPI when scanning.
Bytes = DPI2 x width x height x 3
Note that because DPI is squared, doubling this number will quadruple the size of your file. A scan that produces a 20 megabyte file at 300 dpi will produce an 80 megabyte file at 600 dpi. Likewise, doubling the area being scanned will also increase the file size by a factor of four times.
For example, suppose we are scanning an 8x10 inch photo at 300 pixels per inch. For every square inch we have 300 x 300 pixels. That's 90,000 pixels, and at 3 bytes each, we have 270,000 bytes of information for every square inch.
If we plug the numbers into the formula, we have:
Bytes = (300)2 x 8 x 10 x 3
Solving the equation, we have 21,600,000 bytes or 21.6 MB.
