Rule of thumb to avoid photographing people FEB 18 2007
Rule of thumb to avoid photographing people with their eyes closed: divide the number of people by three (or by two if the light is bad). That means that if you're taking a photo of 12 people, you need to take at least 4 photos to have a good chance of getting a photo with everyone's eyes open. (via photojojo)
Update: Jeff writes: "Way back when we only used film I learned you could tell before looking at the photo whether someone blinked by asking them what color was the flash. If it was white or bluish white, then their eyes were open. If it was orange, then their eyes were closed and they had 'seen' the flash through their eyelids."