You probably noticed that when you opened your left eye, the spot of light you saw through the hole seemed to shrink. Then, when you closed your left eye again, the spot seemed to return to its original size. If you tried switching eyes, you also noticed that the effect was the same no matter which eye was looking through the pinhole. Why does this happen?
Light enters your eye through a hole called the pupil. The colored part of the eye that surrounds the pupil (a muscular ring called the iris) changes its diameter to control the pupil’s size, ensuring that an optimal amount of light enters your eye. Contraction of the iris is controlled in the midbrain by a natural reflex called the photopupillary reflex. Like all reflexes, its reaction is automatic—that is, it doesn’t involve conscious thought. And because the midbrain always sends its messages to both irises, the photopupillary reflex occurs in both eyes, even if only one eye is suddenly exposed to a change in light.
When you’re looking through the tube with your right eye and you suddenly open your left eye, the increase in light intensity triggers the pupils in both eyes to reflexively constrict. The reduced field of view results in your brain perceiving that the hole got smaller. Similarly, when you close your left eye again, both pupils dilate, or get larger, allowing more light to enter. Your brain makes sense of the greater amount of light by deciding that the hole has grown in size.