Visual acuity. This, literally, is the sharpness of your vision. Visual acuity is measured by your ability to identify letters or numbers on a standardized eye chart from a specific viewing distance. It’s tested one eye at a time, with the help of a standardized Snellen eye chart. Visual acuity is a static measurement, meaning you are sitting still during the testing and the letters or numbers you are viewing also are stationary.
It is also tested under high contrast conditions — typically, the letters or numbers on the eye chart are black, and the background of the chart is white. Although visual acuity testing is very useful to determine the relative clarity of your eyesight in standardized conditions, it isn’t predictive of the quality of your vision in all situations. For example, it can’t predict how well you would see:
- Objects that are similar in brightness to their background
- Colored objects
- Moving objects
Three major physical and neurological factors determine visual acuity:
- How accurately the cornea and lens of the eye focus light onto the retina
- The sensitivity of the nerves in the retina and vision centers in the brain
- The ability of the brain to interpret information received from the eyes
Only light that is focused on a very small and highly sensitive portion of the central retina influences visual acuity measurements obtained during an eye exam.