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In thinking about my last post, I realized that I had mentioned that all digital editing is destructive. That issue needs to beexplained further especially to anyone involved in restoring scannedimages of old photographs.

Before taking even the first step in restoring old photos, you should understand whathappens when an image is scanned or otherwise digitized. Thedigitization process involves creating a numerical representation of theinformation contained in the original photograph. In both photographicand digitized images a major concern is the clarity or resolution of theimage. In photographs, the discrete grains of the photographicchemicals are so small that absent microscopic examination, the grainsare not usually visible to the unaided eye. However, if a photograph isis greatly enlarged, you can often detect a graininess to the image. Theresolution of the image depends not only on the ultimate size of thefilm grains but also on the camera and lens system. You cannot get highresolution images out of a bad lens. The measure of resolution in aphotograph and of a camera lens is measured in terms of the how closelyprinted black parallel lines can be resolved. The number of lines perinch (or other measure) is called the image's spatial resolution. There are standard resolution testdocuments that determine the actual physical resolution of a camerasystem and of a photograph produced by the camera and lens combination.One of those is the 1951U.S. Air Force resolution test target. Various other systems oftesting the resolution of camera lenses have also been developed, clickhere for another type of chart.
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