Visual Acuity


Visual acuity is a term used to indicate how clear our vision is, or the clarity with which we see. This depends primarily on the focusing ability of the eye, along with how well specific areas of the brain related to vision processing function.

Within the retina, the area of the eye responsible for detecting light, there are 2 forms of light receptors, rods and cones. The rods are responsible for detecting light only, whereas the cones are responsible for detecting colour. Different areas of the retina have different concentrations of each, the fovea has lots of cones. These cones are activated very quickly by different colours and therefore are responsible for our visual acuity.

Light that hits the fovea generates a message that travels in a nerve called the optic nerve, to the brain areas that are responsible for vision. Depending on how well all these areas function together determines our visual acuity.

Visual acuity can be affected by many common problems such as myopia or shortsightedness. In this case the affected individual is only able to see objects close to them, objects far away seem blurred, the visual acuity at distance is low. Problems such as this are readily corrected with specific spectacles or contact lenses to restore the individuals eyesight.

Visual acuity is measured in many clinical settings, most commonly at the opticians. This is done using a snellen eye chart, which is a series of letters that decrease in size starting with the largest at the top. The chart is designed to be viewed from 6 meters away, however there are different variations for smaller rooms. Depending upon which line you are able to read will determine your visual acuity score. Individuals with normal vision have a visual acuity of 6/6 i.e. they can see at 6 meters what normal people see at 6 meters. As your eyesight declines, the denominator, or bottom number, will also fall. For example 6/3 means that you see at 3 meters what a person with normal eyesight would see at 6 meters.

Visit the Opticians Guide, for further information.


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