For maximum efficiency of seeing, the two eyes need to work together as a team. While there are several ways for the eye doctor to determine how well the eyes are teamed,
the cover test is the simplest and most common.

During a cover test, you are are usually directed to focus on
a small object both at distance and close up. The eye doctor covers each of your eyes alternately, while you stare at the target. As the cover paddle is moved from one of your eyes
to the other, the doctor notes how much each eye has to move when uncovered to pick up the fixation target. If
the alignment of your eyes is outside normal limits or you
are experiencing symptoms, the doctor may recommend treatment. The cover test can not only reveal the presence
of an eye turn (strabismus) but a "tendency" for the eyes to turn called a "heterophoria." Prism in your glasses or eye exercises may be prescribed if a heterophoria is present. 

Children especially at risk - A child's brain is quite plastic and is literally able to rewire itself. If one eye is not fixating straight ahead, information from the eye may be "turned off" (suppressed) by the brain. Because of this suppression, children will not report
seeing double and parents are often unaware there is a problem. Amblyopia or
"lazy eye" is the most common cause of visual loss in children. Amblyopia is a failure of proper visual development due to anything that makes one eye preferred by the brain.
Eye turns that are too small to be noticed can be especially problematic. These small
eye turns can also be detected by the doctor when administering the cover test.

Refraction

Refraction is the name given to the bending of light, as it passes from one medium to another. If you have ever half submerged a straight stick into water, you have probably noticed that the stick appears bent at the point it enters the water. This optical effect
is due to refraction. As light passes from one transparent medium to another, it changes speed, and bends. Another example of refraction is the bending of light rays as they
enter your eye from the air. The two major components in your eye that bend light are
the clear front window of the eye called the cornea, and the internal lens of the eye (located just behind your pupil.)

For you to see a distant object clearly, light coming from the object must be "bent" or "focused" precisely on the inside back surface of your eye called the retina. If light rays
from the distant object are bent too much or too little, after they enter your eye, the
image formed on your retina will be blurry. When your eye bends light too much or too little, it is said to have a "refractive error."  That is why the test done by the doctor to measure this error is called a "refraction."

A refraction is essentially a vision test that determines
which corrective lenses will give you the best visual acuity. The corrective lenses will have a focusing power that is
equal and opposite to the amount of refractive error. A nearsighted eye naturally bends or focuses light too much,
so it requires a "minus" lens (-2.00 D.S., etc.) to subtract focusing power. A farsighted eye naturally bends or focuses light too little, so it requires a "plus" lens (+2.00 DS, etc.)
to add focusing power. "Astigmatism" is also a type of refractive error but it is a bit more complicated to explain. If you're curious, you can learn more in the ABC's of astigmatism.

"Near point" refraction

The eye doctor may repeat the refraction with a target that is held closer. So far, all of our discussion about refraction has assumed a test distance of 20 feet or more. What about things closer than 20 feet, like the typical reading distance of 16 inches or so?
The eye needs to bend light coming from a close object even more to focus the object's image sharply on the retina.

In a younger person (less than age 40-45) the natural lens inside the eye has the ability to become rounder, which allows it bend light more. As we age, the eye's ability to focus on closer objects is gradually lost. This condition is called "presbyopia" and it refers to a near reading problem only. Patients sometimes confuse presbyopia with farsightedness
but they are actually two unrelated conditions.

Reading glasses, bifocals or progressive lenses may be prescribed to add the extra focusing power that the eye's natural lens can no longer provide. In progressive lenses and bifocals, this extra focusing power is called an "add" because it literally adds extra plus power to the distance prescription.


"More to follow"

A comprehensive eye exam consists of several tests, in addition to those discussed
above. In our next newsletter, look for "Your Eye Exam - Part II" in which we discuss
some tests and procedures that reveal a great deal about the health of your eyes.