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When you visit an Optometrist for an
eye examination, they will test your vision by several means.
The refraction error is the degree of error
in focusing light by the eye. There are two types of error. They are the
spherical errors and cylindrical errors, which can be corrected by
glasses, contact lenses, or refractive surgery. Spherical errors happen
when the optics of the eye are too strong or too weak to properly focus
light causing all images to appear blurry.
When you visit an Optometrist for an eye examination, they will test
your vision by several means. One way is with the Snellen eye chart,
named after Dutch ophthalmologist Hermann Snellen who developed the
chart in 1862.
The Snellen chart has eleven lines of block letters. The first line
contains only a single letter "E" of enormous size. The following rows
have increase in the number of letters but the size of the letters
decrease in size. You are then asked to cover one eye, and read aloud
each letter on the rows, beginning with the top row. The smallest row
that you can read accurately indicates your visual sharpness in that
eye.
These symbols or letters are known as optotypes. Only the nine letters
C, D, E, F, L, O, P, T, Z are used in the traditional Snellen chart.
The Snellen chart is placed at distance of twenty feet in countries that
use feet as a unit of measure and normal acuity is said to be 20/20. In
countries using the metric system, the Snellen chart distance is placed
at six meters, normal acuity is designated 6/6.
The huge letter "E" on the chart often represents acuity of 20/200. This
value is considered "legally blind. However, if you cannot read this
letter without your glasses, this does not mean that you are blind, in
most circumstances the acuity ratio refer to the best corrected acuity.
The majority of individuals who have moderate myopia cannot read the "E"
when they are not wearing their glasses, however, once they put their
glasses on, they have no problem reading the 20/20 line or 20/15 line. A
person who is legally blind is unable to read the "E" even when they
have on the best possible glasses.
Another method of testing your vision is with a phoropter. This is an
instrument most optometrist or other eye care professional use during
the eye examination to determine your eyeglass prescription. You are
asked to sits behind the phoropter, and looks through it at an eye
chart. Then the optometrist will flip through different lenses, asking
you if this one is better or if the next and so on, until they find the
best possible combination to give you the clearest possible vision.
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