Quote Originally Posted by blansky
Also I photograph a lot of kids. That expression can change in a heartbeat. I can have frames less than a second apart that are nuanced very differently.
If you're familiar with Paul Ekman at Univ of SF, they've shown that facial expressions change even faster than that -- they've had to use high-speed movie cameras, and they train people to watch for these fleeting changes. The training is aimed at detecting lies, though I've been tempted to try and take it for application in portraiture (fortunately, no one ever lies to me).