Factor Affecting Clarity of Sight

Eye is very sensitive in the right conditions (e.g. clear air, good light, etc.), has approx 1.2 million nerve cells leading from the retinas to the area of the brain responsible for vision, while there are only about 50,000 from the inner ears - making the eye about 24 times more sensitive than the ear.
Visual acuity is the ability of the eye to discriminate sharp detail at varying distances.
Visual acuity is affected by:
(a) Physical factors such as:
•     physical imperfections in one or both eyes (short sightedness, longsightedness),
•     age.
•     The influence of ingested foreign substances such as:
•     drugs, medication, alcohol, cigarettes
(b) Environmental factors such as:
•     amount of light available clarity of the air (e.g. dust, mist, rain, etc.).
•     Factors associated with object being viewed such as:
•     size and contours of the object,
•     contrast of the object with its surroundings,
•     relative motion of the object,
•     distance of the object from the viewer,
•     the angle of the object from the viewer.

An individual with an acuity of 20/20 vision should be able to see at 20 feet that which
the so-called ‘normal’ person is capable of seeing at this range. 
It may be expressed in metres as 6/6 vision. 
The figures 20/40 mean that the observer can read at 20 feet what a ‘normal’ person can read at 40 feet
Long sight - Farsightedness/ Hypermetropia - is caused by a shorter than normal eyeball
which means that the image is formed behind the retina. 
If the cornea and the lens cannot use their combined focusing ability to compensate for this, blurred vision will result when looking at close objects.
Short sight - Nearsightedness/ Myopia - is where the eyeball is longer than normal, causing the image to be formed in front of the retina. 
If the accommodation of the  clens cannot counteract this then distant objects are blurred.
Blind  Spot occurs at the point where the optic nerve enters the retina.
Vision canbe improved by increasing the lighting level.

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