When you're manually (albeit digitally) laying down gradations and layers, you quickly learn how much black you have to put into something to make it look white, and how much white you have to put into something to make it look black. The optical illusion up above, which has recently gone viral and is shocking to anyone who's never done an ID rendering, is an excellent example. The top chiclet is black and the bottom is white, right? Well, not if we look at it after masking off most of the drawing:
Yep, the top and bottom chiclets are in fact the same exact shade of grey. It's the highlights, shading, drop shadow and that junk in the background that fools your eye into assigning different values to it. While they never taught us this in ID school, the actual name of this phenomenon is the "Craik-O'Brien-Cornsweet illusion." Taken by the illusion, Slate even dug up a video demonstration of it:
That image above that's gone viral, by the way, is the work of Dale Purves. Purves is no ID'er; the man is Director of the Neuroscience and Behavioural Disorders program at Duke-NUS Graduate Medical School. His Purves Laboratory studies "visual perception and its neurobiological underpinnings."
Am hoping some of you Photoshop renderers will comment with links to renderings demonstrating your Craik O'Brien Cornsweet skillz.