The human eye: a living lens

Look up from this line of text to something out the window. For a moment the image was sharp here; an instant later, out there. You moved no lens in any mechanical sense; the inside of your eye changed shape. This ability is called accommodation, and it is one of the fastest, most-used "motors" in the body.

Two lenses, one job

Contrary to the school shorthand, most of the light-bending is done not by the lens but by the cornea — the transparent front wall of the eye. It provides about two-thirds of the optical power and is fixed. The lens supplies the rest — but unlike the cornea, it can change that power.

The image must form exactly on the retina — the screen on the back wall of the eye. Because the lens-to-retina distance is fixed, the eye cannot focus like a camera by moving the lens. Instead it changes the focal length of the lens itself.

How the eye changes focal length

The lens is surrounded by a ring of ciliary muscle. When you look far away, the muscle relaxes, the fibres pull taut, and the lens is flattened and "weak". When you want to see something close, the muscle contracts, the fibres slacken, and the elastic lens bulges on its own, becoming rounder and stronger.

The closer the object, the more power P is needed — and the more the lens must bulge. Drag the slider on the figure: as you bring the object closer, the lens swells and the image still lands on the retina. Only when you pass the "near point" can the lens no longer cope and the image blurs.

The fastest autofocus

A camera focuses by moving glass. The eye focuses by changing its shape — with no moving part.

A full shift of focus from far to near takes a healthy eye a fraction of a second and happens reflexively, tens of thousands of times a day. No camera focuses this way — liquid lenses of variable shape that mimic the eye are still a young branch of technology.

Where the model ends

We drew the eye as two thin lenses on an axis. In reality the cornea and lens have complex, non-spherical surfaces, and the image on the retina is inverted — the brain "turns" it back. Refractive errors come mainly from a mismatch between eyeball length and the power of the optical system.

A simplificationWe left out the aqueous humour, the vitreous body and aberrations — but the core is exactly this: a fixed eye length plus a variable lens equals a living autofocus.