What the Tests at Your Eye Exam Do: Refraction, Visual Field and OCT

A full eye exam is really a set of short tests, and each one answers a different question. Some check how clearly you see and what prescription you need. Others check the health of the eye itself, including the optic nerve and retina, where diseases like glaucoma and diabetic eye disease start long before you notice anything.

Patients often tell me, "I had no idea what half of those machines were for." So here's a plain-language tour of the exam, in about the order you'll go through it at Bolt Eye Group.

Why does my eye exam have so many parts?

Because "seeing well" and "having healthy eyes" are two different things. You can read the bottom line of the chart and still have early glaucoma. You can have perfectly healthy eyes and still need glasses. A comprehensive exam checks both. A school or DMV screening only checks the first one, which is why it can't replace an exam. Our post on how often you should get an eye exam goes into why yearly visits matter.

Visual acuity: reading the chart

This is the part everyone knows. You read letters from a chart, one eye at a time, to measure how sharp your vision is at distance and up close. The result is written as a fraction like 20/20 or 20/40. Our post on what 20/20 vision means explains those numbers.

Refraction: the "one or two?" test

Refraction is how we find your exact prescription. You look through a device with many lenses while we flip between choices and ask which is clearer. Small differences matter here, especially for astigmatism, where the angle has to be just right.

A few honest tips from the exam chair:

• There are no wrong answers. "They look the same" is a useful answer.
• Take your time. If two choices are close, say so and we'll show them again.
• Tell us how you use your eyes: lots of driving at night, long hours on a laptop, reading music, golf. It changes what prescription will feel best.

Refraction also tells us if your prescription has changed, which can be an early clue to other things, like blood sugar swings in diabetes or a cataract starting to form.

Eye pressure: checking for glaucoma risk

We measure the pressure inside your eye, called intraocular pressure. Higher pressure is one of the main risk factors for glaucoma, a disease that slowly damages the optic nerve. Depending on the instrument, this may be a gentle puff of air or a quick touch with a small tip after numbing drops. Neither one hurts.

Pressure alone doesn't diagnose glaucoma. Some people have glaucoma with normal pressure, and some have higher pressure without damage. That's why we look at the optic nerve directly, too. Learn more on our glaucoma page.

Slit lamp exam: looking at the front of the eye

The slit lamp is the microscope you rest your chin on. With a bright, thin beam of light, we look closely at your eyelids, lashes, the white of the eye, the cornea (the clear front window) and the lens behind it. This is where we find dry eye, blocked oil glands, scratches, early cataracts, and problems from contact lenses.

Retinal photos and OCT: looking at the back of the eye

The retina is the light-sensing layer at the back of the eye, and the optic nerve carries its signal to the brain. This is the one place in the body where a doctor can see blood vessels and nerve tissue directly, without surgery.

• Retinal (fundus) photography takes a wide, detailed picture of the back of the eye. We keep it on file, so each year we can compare and spot small changes, like a new spot, bleeding from diabetes, or a change in a freckle on the retina.
• OCT (optical coherence tomography) is a scan that uses light to show the layers of the retina and optic nerve in cross-section. It can show thinning of the nerve fibers from glaucoma, or swelling and fluid in the macula (the center of your vision), sometimes years before you'd notice a change.

Both scans are quick, painless and use light, not radiation. Our post on what an eye exam can find explains how these images can reveal diabetes, high blood pressure and other health problems.

Visual field test: checking your side vision

A visual field test maps your side (peripheral) vision. A common version has you look at a center light and click a button whenever you see a small flash somewhere off to the side. It takes a few minutes per eye.

Glaucoma usually takes side vision first, so slowly that people don't notice until a lot is lost. A visual field test catches those gaps early. We also use it to check the path from the eye to the brain, and for patients taking certain medicines that can affect the retina.

Corneal topography: mapping the front of the eye

Topography makes a detailed color map of the cornea's shape. It's essential for fitting specialty lenses like scleral lenses and DreamSculpt™ Ortho-K, and it screens for keratoconus, a condition where the cornea thins and bulges. Our post on keratoconus and scleral lenses explains why catching it early matters.

When should you see an eye doctor?

Book an exam if:

• it's been more than a year since your last one
• your vision has changed, or your glasses don't feel right anymore
• you have diabetes or high blood pressure, or glaucoma runs in your family
• your child fails a school screening, or struggles with reading

Call us right away or go to the emergency room for sudden vision loss, a curtain or shadow over your vision, new flashes and floaters, or a painful red eye with blurry vision.

How we do it at Bolt Eye Group

At Bolt Eye Group, every patient gets corneal topography and retinal photos for a $39 imaging fee. Topography screens everyone for keratoconus, which now has an FDA-approved treatment. The retinal photos let your doctor confirm that your retina and optic nerve are healthy, and show you what they see. We keep them on file to compare year to year. We add visual field testing when glaucoma or a nerve problem is on the table. Then we take our time on refraction, because getting the prescription exactly right is what makes your glasses or contacts feel effortless.

In-office telehealth is available at Canton, East Cobb, Lawrenceville, Riverdale and Johnson City. With an in-office telehealth visit, the same $39 also covers two OCT glaucoma scans, one of the optic nerve and one of the macula. A doctor reviews your scans with you live on a large screen. You also take all your images home free on the I-Scan Platinum Lifetime Baseline, a credit-card-size USB drive. Read how it works on our in-office telehealth page.

We see patients at Canton, East Cobb/Marietta, Lawrenceville, Riverdale and Johnson City, TN. We accept VSP, EyeMed, Davis Vision, Spectera, Medicare and major medical plans, including Blue Cross Blue Shield, Aetna, UnitedHealthcare, Cigna and Humana.

Frequently asked questions

How long does a comprehensive eye exam take?

Plan on about an hour. The tests themselves are quick; most of the time goes to refraction, talking through what we found, and choosing glasses or contacts if you need them.

Do any of the eye exam tests hurt?

No. The air puff can be startling, and dilating drops may sting for a few seconds, but nothing in a routine exam is painful.

Is OCT the same as an X-ray?

No. OCT uses light, not radiation. It's a fast, painless scan that shows the layers of the retina and optic nerve in fine detail.

Why do I need a visual field test if my vision is 20/20?

Because 20/20 only measures your central vision. Glaucoma and some nerve problems take side vision first, and a visual field test is how we catch that early.

Do I need to be dilated?

Sometimes. Dilation widens the pupil so we can see more of the retina. Retinal imaging gives us a great view for many patients, but we'll recommend dilation when your history or findings call for a closer look.

Book an exam at your nearest Bolt Eye Group office

Want a thorough look at both your vision and your eye health? Book an exam at Bolt Eye Group in Canton, East Cobb/Marietta, Lawrenceville, Riverdale or Johnson City, TN. Find your office and book online.