There are two main issues involved in scanning photographs: input (scanning) resolution and output resolution.
INPUT (SCANNING)
When purchasing a scanner, your first instinct may be to buy the scanner with the highest resolution possible. That's understandable -- the higher the resolution, the truer the image, and don't you want the best for your family photos?
Well, yes and no.
What you really want is the highest possible manageable resolution when scanning pictures. Why? One reason is because the higher the resolution, the more pixel depth information that is stored – resulting in larger files. Much larger files. Files that may not be able to be edited with your software -- if you don't have a lot of memory in your PC and if you’re using Photoshop, a 30MB file could crash your software.
(A good rule of thumb for Photoshop users: You'll need three times the amount of free computer memory as the size of the file. For example, if you have a 30MB file, you’ll need to have at least 90MB RAM free to be able to manipulate a photo.)
On the other hand, the less information -- the smaller the files -- the fewer the tonal (color) values, so that the result may not be true to the original. You can also get jagginess, meaning that curves won't be smooth. So a compromise is necessary.
In my opinion, the absolute minimum anyone should scan is at 300dpi. However, this is assuming that you are going to keep the image of your photo at the exact same size. If you are scanning a little 4x5-inch photo at 300dpi and want to create an 8x10 print, then you will have tonal and pixelation problems. To minimize these enlargement problems, you can scan the original at 600 or 800dpi.
In fact, my rule of thumb is that it’s better to scale down than to scale up. Scanning an image at 150dpi and then increasing the image to 300 dpi results in a very bad image. However, scanning the image at 600dpi and then saving it to a 150dpi file (to, say, put on a Web site) won't hurt the quality at all.
OUTPUT RESOLUTIONS
Output resolutions are independent of the scans. They are dependent on the medium and output device. You will want a lower resolution for an image that will appear on the Web, because you need a smaller file size. (Why? Because higher resolutions simply don't show up on monitors. In addition, the larger the file, the longer it’ll take to download or be viewable on the Web.)
YOU SAY INTERPOLATE, I SAY OPTICAL
Be aware that when scanner manufacturers are touting their wares, they may speak in terms of interpolated resolution rather than optical resolution. They may imply that these two terms are interchangeable, but they're not.
Optical Resolution is the actual measure of the amount of data captured when a scan is performed. Interpolated resolution, on the other hand, is the process of the scanner software increasing the image’s resolution based on image pixel values. The software estimates the new values and adjusts for them. This often results in softer images and is not an accurate way to determine the quality of the hardware. As a result, you should disregard interpolated resolution when buying a scanner.
