Why Your Prescription Doesn’t Tell You How Well You Actually See

The other day, a patient came to my office and was visibly unsettled. The optician had told her she had a prescription of -1.5 diopters in both eyes. At the same time, she was told that her visual acuity was only 60 percent. Her question was obvious: Is there something wrong with my glasses?

It is precisely these kinds of situations that repeatedly cause confusion. This is because many people think that diopters and visual acuity mean the same thing. But that is not the case.

What visual acuity really means

Visual acuity is also called visual sharpness. It describes how well the eye can recognize details.

A visual acuity of 1.0 corresponds to 100 percent and is considered the standard benchmark for good vision. It’s important to note: This 100 percent is not a maximum, but a comparative value.

So if someone has only 60 percent visual acuity, that doesn’t automatically mean their glasses are wrong. It just means that the eye currently doesn’t see as clearly as an eye with a visual acuity of 1.0.

What diopters actually mean

Diopters indicate how much a pair of glasses must refract light so that the image lands precisely on the retina.

They therefore describe the necessary optical correction. However, they do not directly indicate how well someone actually sees in the end.

You can think of it roughly like this: Diopters bring the image into focus, but visual acuity shows how well the entire visual system actually works.

Why the two are not the same

Two people can both have the same prescription and still see differently. The reason is simple: glasses only correct part of the problem.

How well vision actually is in the end also depends on other factors, such as the retina, the optic nerve, and how the brain processes the information.

That is why it can happen that someone with correctly prescribed glasses still does not achieve 100 percent visual acuity.

What you can take away from this

Diopters indicate how much correction is needed.

Visual acuity indicates how well you actually see in the end.

If you understand this difference, you can better interpret eye exam results and won’t immediately worry when you see a number like “60 percent.”