Presbyopia is the eye's diminished power of accommodation that occurs with aging. The most widely held theory is that it arises from the loss of elasticity of the crystalline lens, although changes in the lens' curvature from continual growth and loss of power of the ciliary muscles (the muscles that bend and straighten the lens) have also been postulated as its cause.

There is in fact considerable confusion in articles and even textbooks over how the focusing mechanism of the eye actually works. In the classic book, 'Eye and Brain' by Gregory, for example, the lens is said to be suspended by a membrane, the 'zonula', which holds it under tension. The tension is released, by contraction of the cilliary muscle, to allow the lens to fatten, for close vision. This would seem to imply that the cilliary muscle, which is outside the zonula must be circumferential, contracting like a sphincter, to slacken the tension of the zonula pulling outwards on the lens. This is consistent with the fact that our eyes seem to be in the 'relaxed' state when focusing at infinity, and also explains why no amount of effort seems to enable a myopic person to see further away. Many texts, though, describe the 'ciliary muscles' (which seem more likely to be just elastic ligaments and not under any form of nervous control) as pulling the lens taut in order to focus at close range. This makes no sense, as tension would make the lens thinner, focusing further away.

Recently, offers of corrective surgery, and reports of trials have appeared on the internet, which seem to confirm the 'elastic ligament' model, and cast some doubt on the 'hardening lens' theory. Some claim to be able to cure presbyopia by stapling the zonula to restore lost tension against which the ciliary muscles can then act more effectively. Others appear to use laser treatment in an attempt to strengthen or shorten the zonula. However, the fact that many people also go 'long-sighted' with age, seeing in effect, 'beyond infinity', and requiring positive corrective lenses for even distance vision, suggests that the lens does indeed thin with age, though it may still retain more flexibility than is commonly supposed.