The eye sight chart is a device that helps measure subjective visual acuity.

Letter Chart

At a distance, the amount of accommodation is less and the refraction is more accurate. Besides, small changes in the subject’s position do not have a huge impact on the measured acuity.

To measure visual acuity, the doctor measures each eye separately. The right eye is measured first, while the left eye is measured later.

Most charts use a Snellen eye chart with seven lines and smaller letters, each representing better than 20/20 vision. However, there are some that use shapes and numbers.


When calculating the acuity, the physician uses a plus or a minus sign to represent the visual acuity of the eye. In other words, a minus sign represents the number of letters a patient misses on a line, whereas a plus sign refers to the number of letters the patient is able to read on the next line.

Vision Screening

During a vision screening, an eye doctor will use an eye chart to determine the acuity of each eye.

Acuity Ratio

The letter chart also provides an acuity ratio, which measures the best corrected acuity. This test is done while a person wears eyeglasses or contact lenses.


It may be repeated to determine how the individual’s vision is affected by glasses or contacts.

Indirect Eye

The indirect eye sight test is an essential part of physical examinations and standard eye tests. This test helps diagnose eye disorders. Sometimes, doctors order this type of test if they suspect a retinal detachment. It can also be ordered if blood vessels are affected in the eye. However, the results of an indirect eye sight test may vary from one person to another.


The magnification of the fundus depends on the dioptric strength of the convex lens and its position in relation to the eye.

Sight Test

Another indirect eye sight test involves the use of a convex lens to observe the fundus. For example, a 20-diopter lens will produce an image three times greater than the natural vision of a person with 20/200 vision.


During the test, a person may be asked to hold up a number of fingers, and the examiner will record these numbers.


The test determines how much of a correction an eyeglass prescription is needed for each eye.

Refractive Error

It can also determine the type of refractive error a person has, such as myopia or hyperopia.

The test can be performed using either binocular or monocular equipment.