The Snellen eye chart examination tests visual acuity, based on the smallest letters a person can read on the chart when positioned 20 feet away from it, according to MedlinePlus. A 20/20 test result is normal. Eyeglasses or contact lenses can correct abnormal results.
Testing with the Snellen eye chart helps reveal visual problems and symptoms of medical conditions that may require other diagnostic tools, explains MedlinePlus. A patient takes the test with both eyes open, covers one at a time, and reads the lines on the chart aloud. Patients who wear corrective lenses take the test with and without their lenses. A 20/40 result means that the line a patient reads at 20 feet away can be read by a person with normal vision from 40 feet away.