|For the Beginner
The question for the scanning beginner at this point might be "I know it doesn't look perfect, so what? Why not make the final scan and make tonal and color corrections in Photoshop? It's a lot easier doing it there than spending all that time screwing around with the scanner, isn't it?" Or more generally the question is "When do I stop scanning? Or even, how do I know when to stop scanning?" When I started using my first scanner, this was exactly the question in my mind. The books I bought were less than satisfactory (an understatement to say the least) in answering this. What one wants to do is develop a systematic and informed procedure for scanning, make it an almost mechanical process.
In scanning, the name of the game is data capture, quantity as well as quality. Quantity is the scanner's resolution. Quality is in accurately capturing the values of the pixels. Developing an answer starts with the realization that you're dealing with integer quantities.
Look again at the histogram to the left of the link. The horizontal axis spans 256 tonal values; a pixel can only have one of those values. With our current settings, however, our scan would encompass only about 135 values, obviously image detail is already forgone. If one were to bring a picture like this into Photoshop and set the black-white point there, that interval of 135 values is expanded to cover 256 values. The result is that 135 of the 256 values are used. Inevitably some of the intervals increase, some drastically. Thus areas with subtle tonal gradations may develop large jumps from one value to the next and become visually distinct areas. This is also known as "posterization". If you perform the scan so that your scan covers all 256 values, this won't happen. Don't be too inclusive: those 256 values are valuable real estate so you want only the pixels inside the crop or subject area to occupy the range. What you are doing then is setting the picture's black-white point at the scanner level. The next step is to set the color balance. With the tones and colors set globally at the scanner level you can do the final scan.
The rule is: global adjustments at the scanner level; local adjustments in the image editor.