As with taking a digital photo, your intended use for the image determines your best scanning resolution. For example, a 4x6 photograph scanned at 300 PPI will have large pixel dimensions and will appear large on a computer screen, while the same photograph scanned at 72 PPI will have fewer pixels and will appear much smaller on a screen.
Scanning resolution is expressed in terms of DPI or PPI, which you will recognize from our earlier discussion of embedded resolution. The scanning resolution not only determines how many pixels will be captured, it also gets embedded into the image as an invisible piece of information for future output devices. To calculate the pixel dimensions of a scanned photograph, multiply the scanning resolution by the dimensions in inches: Resolution x Inches = Pixels.
Using this formula you can calculate the pixel dimensions, for example, of a 4x6 photograph scanned in using 300 PPI:
300 pixels/inch x 4 inches = 1200 pixels
300 pixels/inch x 6 inches = 1800 pixels
Of course, you could later resize the scanned photograph in FotoFinish, or you could rescan it using a lower resolution (PPI) setting. To calculate the resolution you should use to scan a photo, first decide how big you want your image to appear on your monitor (Pixels / Inches = Resolution). For example, if you want your 4x6 photograph to appear as 400 x 600 pixels on the monitor, then you would scan it in at 100 PPI. If you want to create a 100 x 150 pixel thumbnail, you would scan in your 4-inch x 6-inch photograph at 25 PPI.
Keep in mind that as you increase your scanning resolution you create larger files that might be inappropriately large for E-mailing or Web publishing. Below is a list of some possible resolution and image size combinations: