Thursday, April 26, 2012

Defects of vision and their correction

The main features of the human eye 

  This image is a simplified diagram of the human eye.

Eye anatomy diagram 

   In many respects it is similar to the camera.

An image is formed by the eye lens on the sensitive retina at the back of the eye, whence a message is transmitted to the brain by way of the optic nerve.

The iris

the colored membranous disk,with a pupil in the center. It has

two types of muscular fibers: circular and radial.

In the bright light

 the circular fibers contract and the radial fibers relax: the pupil becomes less in size to reduce the amount of light that enters.

in the less light

the circular muscles relax, and the radial ones contract. The pupil then becomes wider so that more light will enter to facilitate vision.

in other words, the colored portion of the eye automatically adjusts the size of the pupil or circular opening in its center according to the intensity of the light falling on it.

Focusing of the image on the retina is effected by an alteration in the focal length of the eye lens.

This is called accommodation and is brought about by the ciliary muscles, which vary the thickness, and consequently the focal length, of the eye lens.

In this respect the eye differs from the camera, since, focusing the distance of the fixed focus lens from the film.

The eye-ball has a tough white wall called the sclerotic, the front portion “cornea” being transparent.

The space between cornea and lens contains a transparent liquid called the aqueous humor.

The remainder of the eye-ball is filled with a jelly-like substance known as the vitreous humor.

Defects of vision and their correction

The so-called normal eye can accommodate for clear vision of objects from infinity (the far point) down to 25 cm (the near point).

The case of a person with long sight

This person can see objects at infinity but his near point is somewhat further than 25 cm.

True long sight, as distinct from loss of accommodation due to advancing age, is caused by the eye-ball being too short.

Thus if a printed page is held at the normal 25 cm from such an eye it will appear blurred since, for this distance, the eye lens can form a sharp image only at a point behind the retina.

Correction of the long sight defection

Correction is effected by a converging spectacle lens which converges the ray entering the eye just sufficiently to make them appear as though coming from the eye’s own more distant near point.

The case of a person with short sight

In the case of simple short sight the eye-ball is too long so that the effectively parallel rays from a point on a very distant object are focused in front of the retina.

The actual far point for such an eye may be only a meter or less away.

Correction of the short sight defection

Correction is obtained by a diverging spectacle lens which diverges the rays entering the eye so that they appear to be coming from the eye’s own far point.

Long and short sighted correction