A proper scan is the first and most important step toward a good photo restoration (or copy of a print for which the negative is lost). A scan that faithfully reproduces a faded photograph is rarely useful.The upper photo in Figure 1 is a “normal” 8-bit scan from a faded black-and-white print.This is not a good basis for a restoration.The upper histogram in Figure 2 shows why. Only half the total range of values available is actually being used in this scan. A good photograph almost always has a full range of tones from black to white (or nearly so). A good scan of a photograph (whether it’s deteriorated or not) should span the range of values from near-black to near-white.
A high-quality scan’s histogram will look like the middle one in Figure 3; you don’t want it to look like the top one (which makes poor use of the range of available values) or bottom one (which loses some of the tones near white and black). The bottom photo in Figure 1 is from a corrected scan that produces an image that had a much more complete and neutral range of tones. I could expand the tonal range of the normal scan in Photoshop to produce similar contrast and color, but if I do that I get one of those “picket-fence” histograms (Figure 2, bottom). There are many gaps in the tonal scale that will show up as discontinuous-tone steps in the print (Figure 4, left). Compare that with Figure 4, right, enlarged from my corrected scan.