Keratometry

What is keratometry?

Keratometry is a test eye care specialists use to measure the natural curve of the corneas in your eyes. The corneas are the clear, dome-shaped covers at the front of your eyes.

What does keratometry test for?

Keratometry tests your eyes for astigmatism. Astigmatism is a type of refractive error — an extremely common eye condition that causes blurred vision.

Usually, eyes are round, like a baseball. If you have astigmatism, your eye is shaped like a football or the back of a spoon. It happens when your corneas are curved more than usual.

How does keratometry work?

Keratometry measures your corneas’ natural curve by having you look at a series of images. It’s a quick, painless test that your eye care specialist can do in their office. You won’t need a special or separate appointment.

You might have a keratometry test during your regular eye exam or separately if your eye care specialist thinks you have astigmatism. You may also need a keratometry test when you’re getting fitted for contacts or before you have cataract surgery.

Your eye care specialist will use a tool called a keratometer to do the test. It looks a bit like a telescope. But instead of looking at faraway stars, your provider will use the keratometer to examine your eyes up close. A keratometry test follows these steps:

• You’ll rest your chin on a support and your forehead against a guide on the keratometer. Your eye care specialist will adjust the keratometer’s position to line it up with one of your eyes.
• You’ll look through an eyepiece on the keratometer (sort of like how you’d look through a telescope). You may have to cover the eye that’s not being tested.
• Your eye care specialist will ask you to look at an image inside the keratometer. It’s usually a series of three circles or targets with plus signs and minus signs on opposite sides.
• Your eye care specialist will adjust the keratometer to move the targets around until the plus signs line up and touch.
• Your eye care specialist will note the measurements, then reset the keratometer and measure your other eye.

What are the disadvantages of keratometry?

There are no risks to having a keratometry test. The light in the keratometer may be bright to look at, but it won’t hurt your eyes or vision.

The one downside of keratometry is that it’s technically only an estimate of your corneas, not an exact measurement. This is because the test measures your corneas like they’re perfectly round spheres. Because everyone’s eyes have slight, natural differences, your corneas aren’t exactly spheres.

But this doesn’t mean keratometry is useless. Think about a globe that shows the surface of the whole world as perfectly round. We know Earth’s surface is covered with mountains, oceans and other geographic features, so it’s not smooth. And it’s also not exactly, perfectly round. But a globe is still a great tool to help you understand where in the world you are.

Keratometry is the same way. Even though it’s not measuring your corneas exactly, it’s still a useful tool to diagnose astigmatism quickly and accurately in most people.

Some eye care specialists use corneal mapping (corneal topography and corneal tomography tests) instead of keratometry. These tests are more technologically advanced and are becoming more common as ways to check for astigmatism. If they do use keratometry, your eye care specialist may double-check the results with a corneal mapping test.

What are keratometry readings?

Keratometry readings are the results of a keratometry test. You might see this referred to as a K reading. Eye care specialists measure it with units known as diopters.

What is a normal K reading?

A typical cornea bends light at around 43 diopters. Steeper corneas have higher numbers, and flatter corneas have lower numbers. If your cornea is round, it’ll have one consistent K value across its whole surface. If you have astigmatism, your cornea will have two different K values. One for the steepest section, and another for the area where your cornea is flatter. A keratometry test will show this difference, and where the steep and flat sections are.

K readings are usually more for your eye care specialist to note the curve of your corneas. The readings will help your specialist diagnose what’s causing changes in your vision. They’ll also use your K reading to choose the right intraocular lens if you need cataract surgery.

These results and numbers can be confusing. An optician or your regular eye care specialist will help you understand how to read your eye prescription. They’ll also help you find the best glasses, contacts or other vision correction options for you.

When should I contact my eye care specialist?

Your eye care specialist will tell you how often you’ll need regular eye exams.

Astigmatism can change as your eyes naturally change shape throughout your life. But big or sudden changes in astigmatism can be a sign of an issue like keratoconus that needs treatment right away. Visit your eye care specialist if you notice any changes in your eyes or vision — especially if your vision is noticeably worse or blurrier than usual.