Circle of Confusion

Vincent Bockaert, 123di.com

This term usually brings up "circles of confusion" around people's eyes. But this does not need to be the case as it is actually rather simple. Depth of field defines the distance range where things have an acceptable level of sharpness. Although sharpness is very subjective, it is in general based on an 8" x 10" print viewed from a one feet distance. You can, for instance, define that an 8" x 10" print is sharp until you can still distinguish 4 lines per mm. That would represent dots of 0.25mm each or 100 dots per inch (DPI), a fair approximation. Other areas of the image would of course be sharper. In other words, 0.25mm or 250µ (micron) is the cut-off point where we decide things are no longer sharp and is called the Maximum Permissible Circle of Confusion. An 8" x 10" print measures 203mm x 254mm and has a diagonal of 325mm, while 35mm film measures 36mm x 24mm and has a diagonal of 43.27mm or 7.5 times smaller. Since 35mm film needs to be enlarged 7.5 times to obtain an 8" x 10" print from it, the diameter of the Maximum Permissible Circle of Confusion must be 7.5 times smaller or 0.25/7.5 = 0.033mm. If you use 8" x 10" large format film, then the CoC remains at 0.25mm as the information on the negative does not need to be enlarged to create an 8" x 10" print.

This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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