Which structure of the eye primarily controls light entry?

The pupil is the structure of the eye that primarily controls the entry of light. It is the opening in the center of the iris, through which light travels before reaching the lens and the retina. The size of the pupil can change in response to various factors, such as changes in lighting conditions or emotional responses, allowing it to dilate (enlarge) in low light to let in more light or constrict (shrink) in bright light to limit light entry.

While the iris surrounds the pupil and regulates its size by contracting or relaxing, the pupil itself is the actual opening that determines how much light enters the eye. The lens and cornea also contribute to focusing light but do not control the amount of light that enters like the pupil does. An understanding of this is crucial when studying the functions of different eye structures in relation to vision and light perception.