Tag Archives: eye common problems

Presbyopia

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).

The near response of the eye
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.
Eye condition :presbyopia
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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How the eye works

Anatomy of the eye

Below is a narrated animation of eye anatomy and common defects. Click here  to license this video (and other related videos) on Alila Medical Media website.


The eyeball is roughly a sphere of about one inch in diameter. The main components of the eye include:

– The cornea – the transparent front part of the eye. The cornea refracts light and accounts for about two-thirds of the eye’s total focusing power.

– The iris – the pigmented part of the eye that makes up the eye color. The iris regulates the amount of light that enters the eye by adjusting the size of the pupil – an opening in the center of the iris.

– The crystalline lens – a clear biconvex structure located behind the pupil and helps to focus light further. The lens is capable of changing its shape to accommodate near vision.  

Eye anatomy labeled diagram
Fig. 1 : Anatomy of human eye. Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

 

 

 

 

Light refracted by the cornea and the lens creates an image of the visual object on the retina. The retina is a light-sensitive tissue lining the inner surface of the eye. Within the retina, optical information is converted into neural action potentials which are then transmitted to the visual cortex of the brain through the optic nerve.

– The fovea (fovea centralis) is the (small) central area of the retina where the sharpest central vision is achievable.

Common eye defects

In the normal eye, light rays converge right on the retina. This results in sharp vision.
Myopia and hyperopia

Fig. 2 : Light focusing in normal vision (upper panel), hyperopia (middle panel) and myopia (lower panel). Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

 

 

 

 

In myopia, or nearsightedness, a condition in which the eyeball is too long, light rays converge before they reach the retina. The focal plane is located in front of the retina resulting in blurry vision. This happens when the person is looking at faraway objects. Myopia is corrected with concave lenses which diverge the light rays slightly before they enter the eye (Fig. 2). 

In hyperopia, or farsightedness, a condition in which the eyeball is too short, light rays have not yet converged when they reach the retina. The focal plane is located behind the retina resulting in blurry vision. This happens when the person is looking at nearby objects. Hyperopia is corrected with convex lenses which converge the light rays slightly before they enter the eye (Fig.2).

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