Your pupils work sort of like an automatic Iris. When they are exposed to more light, they contract. Sooo you have your camera on your subject, with no light, then flip lights on, and have the camera rolling the whole time. when you bring it into your NLE, just cut it exactly at the point where you turn the lights on. I'm guessing you just want it for a quick cutaway right? Most likely for some kinda weird daron aaronofsky kinda thing. BTW, Pi is a sweet movie.