and how they adapt to different intensities of light. I expect of you used a light meter to measure moonlight reflecting off, say, a white sand beach (a touch of envy here) vs. sunlight reflected off the same beach, the readings would differ by orders of magnitude. Our irises function like camera apertures (or is it the other way around?) to regulate the amount of light admitted to the retina. In moonlight, the iris is wide open. In sunlight, it's very small.