Accommodation reflex

How it works

A camera focuses by moving its lens back and forth. The eye focuses by changing the shape of its lens. The lens hangs inside the eye from a ring of fine fibres. Around that ring is the ciliary muscle³. When the muscle is relaxed, the fibres are tight and pull the lens flat. A flat lens focuses on far objects. When the muscle contracts, the ring gets smaller, the fibres go slack, and the lens springs into a rounder shape. A round lens focuses on near objects².

This lens change is one part of a set of three. When you shift your gaze from far to near, the eyes also rotate inward so that both point at the same spot, and the pupils shrink to sharpen the image¹. All three run together and are known as the near triad. The signal comes from the oculomotor nerve, the third cranial nerve, through its parasympathetic branch¹.

Blurred vision is the trigger. The visual cortex notices that the image is out of focus and passes the message down to the brainstem, which adjusts the lens until the image is sharp¹. It happens in well under a second and you never notice.

The lens stiffens with age. It can no longer round up as far, so the closest point you can focus on drifts away from your face. A child can focus on an object a few centimetres away. By the mid-forties most people hold a book at arm’s length and need reading glasses. This is presbyopia and it is universal⁴.

History and culture

Doctors test the reflex by asking you to look at a far wall and then at a pen held close. They watch for the pupils to shrink and the eyes to turn in. A pupil that shrinks for near objects but not for bright light is a classic sign in neurology, first described in the 1860s by Douglas Argyll Robertson¹.