A visual acuity test is an examination that assesses the clarity of vision in an individual. It tells how well the individual can see the details and shape of a letter or symbol. Visual acuity depends on factors such as:
- The neural functioning
- The sharpness of focus on the retina
- The health of the retina
It is one of the measures of optimal vision. Others include side vision tests, color perception, depth perception and contrast perception. The analysis can be performed by an ophthalmologist, optometrist, optician, nurse or an eye clinician.
There are different types of visual acuity tests. They include:
- Snellen test- The Snellen Chart is the standard test for visual acuity. It has eleven rows with printed capital letters or symbols. The letter chart is the most common of the two. It has the letter ‘E’ in the first row. Subsequent rows have more letters which gradually decrease in size as one goes down the chart. The patient is required to read the letters out loud from a distance of 20 feet.
- Tumbling E / Random E- The test is used in young children to check for visual problems as well as illiteracy. The chart contains the letter ‘E’ in different alignments. The sizes also decrease going down the rows. The patient is required to point the direction of the letters according to how they see.
- Jaeger test- The Jaeger test is used to measure near vision. It includes a small card that is hand-held at a specific distance. The card contains printed blocks of texts in a variety of sizes arranged from top to bottom. The test uses a scale that ranges from J1 to J11.
The visual acuity test is part of the comprehensive eye exam for eye conditions. It is important when checking for changes or defects in vision. Some of the common conditions that may require a visual acuity test include:
- Refractive errors – Also referred to as ametropia, refractive errors are the most common cause of poor visual acuity. The different types of errors are nearsightedness, farsightedness, astigmatism and presbyopia. They are caused by changes in the corneal shape or rigidity of the lens.
- Amblyopia – The condition causes blurry vision, usually in one eye. The symptom can be accurately diagnosed with a visual acuity test on the lazy eye. Another examination can confirm the diagnosis.
- Neural conditions such as macular degeneration and retinal detachment - They are associated with blurry vision. Together with other tests, the visual acuity test can be used for diagnosis of the two.
Preparation & Expectation
There are no preparations required.
The procedure of visual acuity testing is simple. The patient will be required to do the following:
- Remove glasses/contacts (for those who wear)
- Sit or stand at a specific distance from the chart/, usually 20 feet for the Snellen test
- Cover one eye with the hand/piece of paper
- Read out loud the letters or numbers on the chart/card per the rows. The doctor will instruct how to go about it. In the case of the random ‘E’ test, the patient uses their finger to point the direction of the letter ‘E’ from how they view it.
Repeat with the other eye
In some instances, the patient can do it again with glasses or contacts on. At the end of the session, the doctor records the results.
The results depend on the test taken:
- Snellen test – A 20/20 result means normal visual acuity. The top number stands for the distance from the chart. The number below shows the distance at which a person with normal vision can read the same letters/numbers correctly. A 20/40 outcome, for instance, means that the line, read at 20 feet, can be read in the same way by a person with normal eyesight at a distance of 40 feet.
- Random ‘E’ test – A pass in the test means the patient got the correct alignment for the ‘E's in the chart.
- Jaeger eye test – From to the J1 to J11 scale, J2 is said to be the same as the 20/20 vision. Therefore, patients with a J2 score are considered to have the best visual acuity.
The results determine whether the doctor will prescribe corrective lenses or not.
Risks & Complications
There are no known risks or complications associated with the visual acuity test.