Presbyopia - Focusing Mechanism of The Eye

Focusing Mechanism of The Eye

In optics, the closest point at which an object can be brought into focus by the eye is called the eye's near point. A standard near point distance of 25 cm is typically assumed in the design of optical instruments, and in characterizing optical devices such as magnifying glasses.

Without correction, the near point is at 3 inches (7 cm) at age 10, to 6 inches (16 cm) at age 40, to 39 inches (1 meter) at age 60. As a result, a 60-year-old must use corrective lenses to read books or magazines at a comfortable distance.

There is some 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 ciliary muscle, to allow the lens to become more round, for close vision. This implies the ciliary 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 farther 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 to focus at close range. This has the counter-intuitive effect of steepening the lens valve cytokinesis centrally (increasing its power) and flattening peripherally.