Visual field test: what it is and what to expect

A visual field test, also called perimetry, maps how much you can see above, below and to the sides while you look straight ahead. It measures how sensitive each part of your vision is, one eye at a time.

In brief

• What it is: a computer-run test in which you press a button each time you notice a small light.
• What it feels like: painless. It needs concentration.
• How long: around five to ten minutes per eye. Allow about half an hour in all.
• Preparation: come rested, bring your glasses, use your regular drops as usual.
• Why it is repeated: the value lies in comparing tests over time.

Why it is done

A letter chart measures only the very centre of your vision. Several conditions affect the vision around the centre first, where a person does not notice the loss, because the other eye and the brain fill in the gap. The field test finds and measures those areas.

• Glaucoma: the most common reason. Glaucoma usually affects the side vision first, without symptoms. The field test shows whether any vision has been affected and, over repeated tests, whether it is stable.
• People being watched for glaucoma: for example with raised eye pressure, a suspicious optic nerve or a strong family history.
• Conditions of the optic nerve or the visual pathway in the brain: the pattern of field loss helps to locate the problem.
• Some retinal conditions and some long-term medicines: where the doctor needs to know whether the surrounding vision is affected.

What you will experience

1. One eye is covered with a patch. The other is tested first.
2. You sit with your chin and forehead on a support, facing the inside of a white bowl. A lens of your spectacle power may be placed in front of the eye.
3. You look steadily at a small light in the centre. You keep looking at it throughout.
4. Small lights appear briefly, one at a time, in different places and at different brightness. Each time you notice one, you press the button.
5. The machine checks from time to time that you are still looking at the centre.
6. After a short rest, the other eye is tested.

The test is painless. Nothing touches the eye, and there is no puff of air or flash.

Things that help

• You will not see every light. Many are set too dim to see on purpose. Missing them is expected.
• Do not look around for the lights. Keep your eye on the central light and let the others appear in your side vision.
• Blink normally. A good moment to blink is just as you press the button.
• If you are unsure whether you saw a light, do not press. Press only when you notice one.
• You can pause. Holding the button down pauses the test on most programmes, and the technician can stop it if you need a rest.
• Say if you are uncomfortable. The chair and chin support can be adjusted before the test begins.

How long it takes

Around five to ten minutes per eye, depending on the test programme and on how the eye responds, with a short rest between eyes. Allow about half an hour for the whole test, including setting up.

How to prepare

• Come rested if you can. Tiredness makes the test harder and the result less reliable.
• Bring your current glasses.
• Use your regular eye drops and medicines as usual.
• No fasting is needed.
• If you have had a field test before, bring the printout, even from another hospital.
• If you have difficulty sitting upright for some minutes, or holding or pressing a button, tell the staff beforehand.

What the result can tell

• Whether there are areas of reduced sensitivity in your field of vision, and where they are.
• Whether the pattern fits glaucoma or points to another cause.
• Over a series of tests, whether the field is stable or changing. This is what guides glaucoma treatment.

What the result cannot tell

• It depends on you. This is a test of responses, not a photograph. Tiredness, anxiety, poor concentration or an uncomfortable position all affect it. The printout includes reliability measures, and the doctor takes them into account.
• A first test is often not representative. Most people do better once the task is familiar. An abnormal first test is usually repeated before any conclusion is drawn.
• One test rarely decides anything. Results vary somewhat from day to day even in a stable eye. Change is judged across several tests.
• It does not find the earliest damage. Some nerve fibres can be lost before the field test shows a change. This is why the optic nerve is also examined and measured with an OCT scan.
• Other things can reduce the field. Cataract, a drooping upper eyelid, a small pupil or the wrong lens in front of the eye can all affect the result without any disease of the nerve.
• It does not measure eye pressure. Pressure, the appearance of the optic nerve, the OCT scan and the field test are read together.

Visual field testing at the hospital

Dr. P. Siva Reddy Eye Hospital has automated visual field testing on site. The doctor reads each test together with your examination and your earlier tests. More on how glaucoma is monitored is on the glaucoma page.

Frequently asked questions

Is a visual field test painful?

No. The test is painless. Nothing touches the eye, and no drops are needed for the test itself. It asks for concentration, which some people find tiring.

How long does a visual field test take?

Usually around five to ten minutes per eye, depending on the test programme, with a short rest between eyes. Allow about half an hour for the whole test.

I missed many of the lights. Did I fail?

No. The test is designed to find the dimmest light you can see at each point, so it deliberately shows lights that are too dim to see. Everyone misses many of them. There is no pass or fail.

Why do I have to repeat the test?

There are two reasons. First, the test depends on attention, and a first test is often less reliable than later ones because the task is unfamiliar. Second, glaucoma is followed by comparing tests over time, so a set of baseline tests is needed before change can be judged.

Can I wear my glasses for the test?

Bring them. The technician usually places a lens of the right power in the machine in front of the eye being tested, because spectacle frames can block part of the field.

Should I use my glaucoma drops on the day of the test?

Yes. Use all your regular drops and medicines as usual unless the doctor has told you otherwise.

Is the field test done before or after dilating drops?

Usually before, because the test is generally more reliable with the pupil at its natural size. If your visit includes dilation, expect the field test first.

My OCT scan was done. Why do I also need a field test?

They measure different things. The OCT scan shows the structure of the optic nerve. The field test shows how well it is working. In glaucoma the two do not always change together, so both are followed.

Related pages

• Glaucoma
• OCT scan explained
• Glaucoma tests: OCT and visual field compared
• YAG laser, including iridotomy for narrow angles
• Before your visit
• Facilities and equipment
• Patient guide: all pages