28 Oct Visual Acuity – How We Measure Your Vision
When you go to your eye doctor, we check your “vision,” but there are several ways to actually monitor or evaluate your vision. Basically, almost all vision testing is designed to measure the function of your macula. Macular vision is your central vision. It is our most useful vision. When we are 20/20, we are testing central vision only.
“The Eye Chart” measures Snellen acuity. This is the common eye chart that has the big “E” at the top of the chart. Snellen was a Dutch ophthalmologist who developed the chart in the 1800’s. It is the chart on the right. Almost every emergency room and doctor’s office has this chart.
Snellen acuity measures your vision as a “fraction.” We, in the U.S., call 20/20 vision the standard or “perfect” vision.
20/20 means that you are able to see at 20 feet, what “normal” person sees at 20 feet. Another example, 20/50 means that you must stand 20 feet from a target/chart to see the same as the “normal” person standing 50 feet away.
Legal blindness is defined as 20/200 or worse. If the vision is worse than the big “E” we use the following conventions;
- Counting Fingers – measuring your ability to count the examiners fingers at a given distance.
- Hand Motions – vision is limited to seeing movement, such as a waving hand.
- Light Perception – vision is poor, retaining only the ability to see light.
- No Light Perception – nothing, completely black.
Snellen acuity has many limitations, but it has been the basis for testing visual acuity. Snellen acuity can NOT measure distortion, color, glare or blind spots. Snellen acuity is a measure of your macular function, that is, how well your macula works.
The Amsler Grid is a commonly used to self-monitor central vision, especially distortion. It is most often given to patients with macular degeneration. Patients are instructed to place the chart in a common area, for instance, refrigerator door or bathroom. The grid looks like a piece of graph paper. It gets you accustomed to any distortion that may be present. You are looking for new areas distortion. If new areas develop, one should alert their eye care professional. The American Academy of Ophthalmology has a link for an Amsler grid, including directions.
Glare Testing is a slight variation of measuring Snellen acuity. A small light that may cause glare is shining into your eyes while reading the eye chart. Typically, if a cataract is causing glare, it will reduce your acuity when the glare is recreated. Some people actually see well until glare is introduced. A typical scenario where glare becomes problematic is driving at night with oncoming headlights.
A Visual Field measures your peripheral vision. This is usually used to monitor glaucoma, but can be used to diagnose strokes, intracranial tumors or other visual abnormalities. There are several types of visual fields; Humphrey, Goldmann, Octopus.
Potential Acuity Meter (PAM) is a device that projects an eye chart onto your retina. Not all offices use these, but they are very helpful in testing retinal function more directly if vision is decreased, but not sure of the etiology.
Color vision and stereo-acuity are specialized tests and are not routinely used to test vision in adults. There are special images and puzzles designed to measure either. I never utilize these for examining patients either diabetic retinopathy or macular degeneration.
Randall V. Wong, M.D.
Ophthalmologist, Retina Specialist