Scanning and Histograms

Answer 1

 Histograms are used in scanning to show how the values of the pixels that have been captured are distributed over the scanner's tonal sensitivity range, just as a camera's meter indicates how well the image matches the film's tonal range.  In other words, they tell you how efficiently the scanner is being used.  Without them it would be extremely difficult to achieve technically optimal scans.

    
• Histograms provide a graphical display of the state of the image, summarizing the vast quantity of data contained in a typical scan

    
• It is nearly impossible to make the final adjustments through visual feedback alone. Monitors are incapable of displaying the subtle differences in the shadow areas which are easily discerned in a histogram