On 4/17/2015 12:01 PM, Janet Gable Cull wrote:
Richard, will you describe "edge effect"? I wonder if that's what I observed in my friends scans that I like. I wonder how and why it would do that? Does it somehow perceive the light and dark of an image, and treat them differently?Edge effects, sometimes called border effects, are variations in development at the border of a high and low density area. The nature of the effect depends on the developer, specifically on whether its reaction products suppress or enhance development. The effects are very localized because they are due to diffusion of the developer reaction products in the emulsion. The usual effect is to increase the contrast at the edge. Because the effect depends on reaction products its increased where the developer is highly diluted or where there is insufficient agitation to wash away reaction products at the surface and bring fresh developer there. When the effect is not extreme it enhances sharpness. Note that this is sharpness not resolution. Sharpness developers were long popular for 35mm use because they tend to compensate for the lack of sharpness caused by insufficient film and lend resolution. More modern films have better resolution. However, the eye tends to judge sharpness by edge contrast so a high resolution but relatively low contrast image will look less sharp than one with high edge contrast but considerably lower actual resolution. Extreme edge effects can look like someone drew lines around objects. This used to be a serious problem with motion pictures because the release print was often several generations from the original. Each generation increased the edge effects until one got "edge crawl". Edge effects and compensation are related since both depend on local exhaustion of the developer.
