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.

Corrective lenses can magnifier, introduce blur, or correct blur depending on the optical prescription. However, because most people first encounter lenses as magnifying glasses, they naturally think of lenses as magnifiers. However, while corrective lenses do have a slight magnification or a reducing effect. This is an unintentional and usually undesirable side effect. Corrective lenses do not correct vision through magnification, but rather by improving vision by reducing blur.