In thinking about my last post, I realized that I had mentioned that all digital editing is destructive. That issue needs to be explained further especially to anyone involved in restoring scanned images of old photographs.
Before taking even the first step in restoring old photos, you should understand what happens when an image is scanned or otherwise digitized. The digitization process involves creating a numerical representation of the information contained in the original photograph. In both photographic and digitized images a major concern is the clarity or resolution of the image. In photographs, the discrete grains of the photographic chemicals are so small that absent microscopic examination, the grains are not usually visible to the unaided eye. However, if a photograph is is greatly enlarged, you can often detect a graininess to the image. The resolution of the image depends not only on the ultimate size of the film grains but also on the camera and lens system. You cannot get high resolution images out of a bad lens. The measure of resolution in a photograph and of a camera lens is measured in terms of the how closely printed black parallel lines can be resolved. The number of lines per inch (or other measure) is called the image's spatial resolution. There are standard resolution test documents that determine the actual physical resolution of a camera system and of a photograph produced by the camera and lens combination. One of those is the 1951 U.S. Air Force resolution test target. Various other systems of testing the resolution of camera lenses have also been developed, click here for another type of chart.
In the case of a digital image, the "resolution" is really nothing more than a pixel count. Each distinct microscopic pixel is a discrete sensor. The sensor array is made up of rows and columns of pixels. The pixel resolution is represented by two numbers, one for the columns or width and one number for the rows or height. Current technology has numbers of pixels over one million, so the actual number is divided by one million and referred to as "Megapixels." There are other references to pixel resolution that include describing pixels per length unit or pixels per area unit, such as pixels per inch or per square inch. These pixel resolutions are not true resolutions, but are commonly referred to as such; they serve as upper bounds on image resolution. Wikipedia.
So, digitizing a photograph will always end up reducing the amount of information present in the original. No matter how high the resolution of the scan, the discrete nature of the pixel elements will put an upper limit on the resolution and pixel resolution does not presently come close to equaling spatial resolution. Likewise, once the image is digitized, whether from a scanner or directly from a digital camera, any changes to the image will result in a loss of information and hence, all edits are destructive.
Next, digital formats for saving images, is there a RAW image in your future?