Below is a narrated animation of the Near Vision of the Eye. Click here to license this video and other similar images/videos on Alila Medical Media website.
Presbyopia is a very common age-associated condition in which the eye loses the ability to adjust to near vision.
For information on eye anatomy other common defects click here.
How near vision is achieved?
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 (Fig. 1, upper panel).
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 same area of both retinas of the two eyes. Failure of doing so (e.g. when eye muscles are weak) would result in double vision.
– Constriction of pupil – this is to reduce spherical aberration. Spherical aberration occurs when light rays strike on 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 contracted to make the lens thicker, more convex. This increases the optical power of the lens, it now can converge the light rays on the retina (Fig. 1, lower panel).
Fig. 1: The near response of the eye. Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.
Presbyopia and correction
With age, the lens loses its flexibility and becomes stiff. It can no longer change its shape to accommodate near vision. This results in prebyopia – inability to see nearby objects.
Prebyopia is corrected with convex lenses that converge the light rays slightly before they enter the eye (Fig. 2). However, as this is needed only for looking at close-range objects, bifocal lenses are usually recommended.
Fig. 2: Presbyopia and correction lenses. Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.
Presbyopia is not to be confused with hyperopia, a condition in which the eyeball is too short.
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