The pupil of a human eye expands or contracts to admit more or less light on the lens. The lens focuses the light on the retina. The retina is like a screen. Photoreceptors on the retina respond to the images formed and our brain interprets these responses. Retina is gorged with tiny blood vessels that supply blood to the photoreceptors.
Ciliary muscles control the size of the pupil.
When the light is intense, the ciliary muscles contract and the pupil becomes small. When the source of light is dim, the ciliary muscles relax and the size of the pupil is expanded to admit more light in the eye. When a subject is being photographed in low light conditions, the pupil s of the subject are naturally expanded . The flash throws intense light for the duration of the photograph on the subject. This happens in a fraction of a second.