Online vision tests: How accurate are they?

Appearing on the screen are numbers, letters or symbols. The vision test on your web browser or smartphone app checks how clearly you can make them out and then shows you the results. Seeing isn't believing though - at least it shouldn't be.

While the tests can give you a rough idea of your visual acuity and sensitize you to any changes, they can lead to a false sense of security or unwarranted worry, warns Dr Jan Terheyden, an ophthalmologist at Bonn University Hospital (UKB) in Germany.

Neither good nor bad test results allow reliable conclusions about your eye health, he says, especially since they're heavily dependent on factors such as screen brightness, room lighting and viewing distance.

What's more, many eye disorders can long be present without noticeably impairing visual acuity. Disorders such as glaucoma, age-related macular degeneration and diabetic retinopathy can't reliably be ruled out by an online vision test, points out Dr Frank G Holz, medical director of the UKB eye clinic.

What's important, says Terheyden, is to take acute eye problems or sudden vision changes seriously and get a medical examination, which in rare cases can reveal a serious illness.

In short, online vision tests are no substitute for an examination by an optometrist or ophthalmologist, he says. But he doesn't take a totally dim view of them, noting that they can make you more aware of any eyesight deterioration so long as they're well developed, scientifically tested and user friendly.

If you're not sure which tests are reliable and suitable for you, it's best to consult an eye doctor.