Vision tests are not equivalent to eye examination.Vision tests are a part of an eye exam to determine if there is any reduction in the ability to see.Most vision tests check visual acuity that is the sharpness of central vision.Refractive errors,such as myopia(shortsightedness),hypermetropia(farsightedness) and astigmatism are the most common causes of visual acuity.

The first step in the eye screening exam is to assess the persons visual acuity by using either a standardized visual acuity chart or a near card for bedside use.

Snellen test serves as a standard method for checking vision.It's performed with the help of a snellen chart and a phoropter.Snellen chart displays lines of numbers and block letters with the size decreasing from top to bottom.Phoropter is a simple device to hold different lenses infront of each eye.

The patient is positioned at a distance of 20feet (or 14inches if near card is being used) from the snellen chart.The patient is asked to read progressively smaller lines on the chart,the snellen line is determined that he or she can read with greater than 50% accuracy.The lenses in the phoropter are changed until the letters near the bottom of the chart canbe read.

An optometrist determines the refraction by Phoropter by placing oclur lenses with different powers and asking the patient to select lens of the best power.

Each eye is tested separately.After this procedure,the corresponding vision is documented for each eye.Normal value is 20/20 or 6/6 if the distance is in meters.

For example, OD=20/200 and OS= 20/200

OD stands for ocular dexter which represents the right eye. OS stands for ocular sinister which represents the left eye. The numenator of this fraction represents the distance of the patient from the chart.The denominator value shows the distance from which a person with normal visual acuity would read the line with greater than 50% accuracy.

Near vision test is performed only when the standard snellen test is impracticable.It's less less precise than snellen test.This test is similar to snellen test while the optometrist places the near vision card roughly 14inches away from the eyes,repeating the same steps as of snellen test.If the patient has already reading glasses or bifocals ,they shouldbe worn for the exam.

Snellen test maybe unable to use if the patient has vision less than 20/400 which means the patient can see an object from 20 feet what canbe seen by a normal eye at a distance of 400feet.

More primitive tests are needed to approach such cases to test visual acuity.Three tests are performed step by step when the previous one shows no results.

• Counting fingers
• Hand movement
• Light perception

Counting fingers:The distance at which the patient is able to count the fingers held by the examiner is reported.e.g a documented vision CF3feet means the patient is able to count the fingers at a distance of 3feet.

Hand movements:This test is needed if the patient is unable to count the fingers from very short distance.The patient is asked then to distinguish horizontal hand movements from vertical hand movements.The result is documented as 'HM'(hand motions).

Light perception:Finally, if the patient's vision has been affected to an extent that he or she is unable to respond to hand movements.The examiner can test for light perception by shining bright light directly on the patients eye,covering adequately the opposing eye.A positive response is recordedd as LP(light perception;a negative response is recorded as NLP(no light perception)

Having 20/20 vision doesn't assure there couldbe no vision problems.Other tests maybe needed if the patient has a difficulty in seeing.There are some eye charts available online that may help to check out common visionary problems.

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