The near response of the eye
When the eye is focused on a faraway object, light rays coming from the object are almost
parallel and have no difficulty to converge on the retina.
When looking at a nearby object, light rays coming from the object are too divergent to
come into focus on the retina without any help. In order to see nearby objects clearly,
the eye has to make the following adjustments: – Convergence of the two eyes – this is
to make sure the object is focused on the fovea of each retina. Failure of doing so
– for example, when the eye muscles are weak – would result in double vision. This
is because the object is focused on different parts of the two retinas and the brain sees
two images. – Constriction of pupil – this is to reduce
spherical aberration. Spherical aberration occurs when light rays hit the edge of a lens
and produce blurriness. Constricted pupil allows light rays to enter the lens only at
the center where they are best refracted. – Accommodation of the lens – ciliary
muscles contract to make the shape of the lens more convex. This increases the optical
power of the lens. It now can converge the divergent light rays onto the retina.
Presbyopia is a very common age-associated condition in which the eye loses the ability
to adjust to near vision. In presbyopia, the lens loses its flexibility with age and becomes
stiff. It can no longer change its shape to accommodate near vision.
Prebyopia is corrected with convex lenses that converge the light rays slightly before
they enter the eye.