Technique: Noise Reduction Using… Noise?

Digital camera and sensor manufacturers have made great strides over recent years in reducing image noise. Improved light gathering efficiency and processing has yielded dramatic improvements in high ISO performance to the point where one can use ISO 1600 and below with impunity, and generally ISO 3200 to 6400 with relatively little discomfort. In cases of the pro bodies and full frame sensors you can comfortably shoot in low light at insane levels from ISO 12,800 to 25,600 and expect good results (though you’ll pay a hefty price for this ability in the Nikon D4/D4s and Canon 1Dx). So why on earth would you want to add noise to an image in post? And, more paradoxically, use “artificial” noise to reduce image noise? Having been amazed by this technique for some time and recently working on new images/reprocessing old ones where it came into play it struck me as timely to post another quick tutorial.