If you're looking to capture a photo of a group of people in which everyone has their eyes open, you're probably going to be snapping more than one shot in order to hedge your bets. Every family has a couple of chronic blinkers, after all.
So how many should be necessary? According to an equation laid out by two Australian researchers, for low-light photos of less than 20 people, you need to divide the number of people by two and take that many pictures to guarantee that one will have all eyes open. If the photo is taken in a well-lit setting, you can get by with dividing the number of people by three. Factors in the equation apart from the number of people and the lighting conditions are the shutter speed and blink frequency [source: Cheung].