The eye: a natural camera

A converging optical system

The eye works like a camera. Light passes through several transparent media before forming an image on the retina: the cornea, the aqueous humour, the lens, and finally the vitreous humour. The lens is a converging lens whose power can be adjusted by the ciliary muscles: this is known as accommodation.

Image formation

For an object to be seen clearly, its image must be formed precisely on the retina, which acts as a screen. This image is real, inverted and smaller than the object: it is the brain that turns it the right way up when processing visual information.

The role of the iris

The iris regulates the amount of light entering the eye by changing the diameter of the pupil, much like the aperture of a camera:

Situation	Pupil diameter	Role
Bright light	Small (~2 mm)	Limits glare
Dim light	Large (~8 mm)	Captures more light

The range of clear vision

An emmetropic (normal) eye sees clearly from the punctum proximum (PP, approximately 25 cm, the minimum distance for clear vision) to infinity (punctum remotum, PR), without any particular effort at infinity.

Common pitfall: do not confuse the eye with a simple fixed lens. Unlike a webcam lens, the eye constantly accommodates to maintain a clear image at different distances.