Visual Field Test (Perimetry): Why It's Done and What the Results Mean

You can read the bottom line of an eye chart perfectly and still be losing vision. Glaucoma takes the edges first, and the brain covers the gap so convincingly that most people notice nothing for years. The visual field test is how that loss gets measured before it becomes obvious.

Key Takeaway

A visual field test, also called perimetry, maps how much you can see across your whole field of vision straight ahead and out to the sides one eye at a time. It is used mainly to detect and monitor glaucoma, and also to investigate retinal and neurological conditions. The test takes a few minutes per eye, involves no drops or injections, and is painless.

What Is a Visual Field Test?

Your visual field is everything you can see while looking straight ahead. In a healthy eye, it extends roughly 100 degrees outwards to the side, 60 degrees towards the nose, 60 degrees upwards and 70 degrees downwards.

Everyone also has a natural blind spot about 15 degrees out to the side, where the optic nerve leaves the eye, and there are no light-sensing cells. You never notice it, because the brain fills it in.

A visual field test measures how sensitive your vision is at many separate points across that area. The result is a map showing where you see well, where you see poorly, and where you do not see at all.

Why Is a Visual Field Test Done?

Glaucoma is the main reason

Glaucoma damages the optic nerve, and it does so from the periphery inwards. The central vision you use to read stays sharp until late in the disease.

This produces a situation patients find hard to believe. In advanced glaucoma, visual acuity can still measure 6/6 on the chart while the peripheral field has been severely reduced. The eye chart cannot detect this. The visual field test can.

One caveat matters. Field defects generally do not appear until a considerable proportion of optic nerve fibres has already been lost. This is why perimetry is used alongside optic nerve imaging and eye-pressure measurement rather than on its own.

Other conditions it helps detect

• Retinal disease, including retinitis pigmentosa and damage affecting the macula
• Optic nerve disorders, including inflammation and ischaemic optic neuropathy
• Neurological conditions a pituitary tumour pressing on the optic chiasm produces a distinctive loss of the outer half of vision in both eyes; a stroke affecting the visual pathway produces loss on the same side in both eyes
• Monitoring certain long-term medications known to affect the retina
• Assessment of visual function where field loss may affect driving or daily activity

Because the pattern of loss maps to a specific point along the visual pathway, the test can indicate roughly where the problem lies, sometimes pointing to a cause outside the eye entirely.

Types of Perimetry Test

Confrontation testing

The simplest version. The examiner sits opposite you, covers one of your eyes, and asks you to count fingers or spot movement in different directions while you look straight at them. It needs no equipment and takes seconds.

Its limitation is significant: it reliably picks up only large defects. It is a screening step, not a substitute for machine testing.

Automated (static) perimetry

The standard clinical test. You look into a bowl-shaped instrument and press a button whenever you see a small light appear. The lights are stationary, appear briefly, and vary in brightness. The machine works out the dimmest light you can detect at each location.

Common instruments include the Humphrey Field Analyzer, the Octopus perimeter and the Humphrey Matrix. Most modern testing uses a smart algorithm that predicts which brightness levels are worth testing, which keeps the test shorter than it would otherwise be.

Goldmann (kinetic) perimetry

Here a light is moved inwards from outside your field of vision, and you signal when you first see it. A trained technician maps the boundaries by hand.

It is less widely available because it needs a skilled operator, but it remains useful for mapping the far periphery and for patients who struggle with automated testing.

How to Prepare for a Visual Field Test

1. Bring your current spectacle prescription. You will not wear your own glasses during the test; the frames create false gaps in the map. A trial lens is placed in front of your eye instead, and the correct prescription must be known.
2. Rest beforehand if you can. Tiredness genuinely affects results. Concentration lapses show up as errors.
3. Mention a droopy eyelid. If your upper lid or lashes block the view, the technician may tape the lid up. Otherwise, it produces a false shadow across the top of the map.
4. Tell the technician if this is your first test. Results often improve on the second attempt simply through familiarity, and knowing it is your first helps with interpretation.
5. Allow more time than the test itself takes. Setup, both eyes and a short break between them add up.
6. Ask before the test starts if anything is unclear. It is much harder to fix a misunderstanding halfway through.

The test does not usually involve dilating drops. If drops are planned for the rest of your examination, the field test is generally done first.

What Happens During the Test

1. You sit with your chin on a rest and your forehead against a bar, looking into a lit bowl.
2. One eye is covered with a patch. Each eye is tested separately.
3. A trial lens is positioned in front of the eye being tested.
4. You are asked to keep looking at a central target and not to move your eyes to search for the lights.
5. Small lights appear briefly at different places. You press the button each time you see one.
6. The same is repeated for the other eye.

Two things trip most people up. First, the lights appear in your side vision; you are meant to notice them without looking at them, which feels unnatural. Second, many of the lights are deliberately too dim to see. Missing them is not failure. It is how the machine finds your threshold.

There is nothing to pass. Trying too hard and pressing when unsure makes the result less useful, not more.

How to Read Your Visual Field Report

A printout is dense. Here is what each part is doing, in the order a clinician reads it.

Start with the reliability indices

Before anything else, the report says whether the test can be trusted. Three numbers matter:

• Fixation losses – how often you appeared to see a light placed in your natural blind spot, which suggests your eye had drifted.
• False positives – how often you pressed the button when no light was shown. Common in people anxious to do well. This is the “trigger happy” pattern.
• False negatives – how often you missed a bright light in a spot where you had already seen a dimmer one. Usually fatigue or a lapse in attention.

Published interpretation guidance treats a field as unreliable when false positives exceed about 15% on modern testing algorithms, or fixation losses exceed about a third. An unreliable field is not interpreted further; it is repeated.

There is also a gaze tracker along the bottom, a row of small marks recording eye and head movement throughout the test, including blinks.

The greyscale map and why not to trust it alone

The greyscale is the picture everyone looks at first: light areas where you see well, dark areas where you do not.

It is also the least reliable part of the report. The greyscale is not compared against normal values for your age, and where the machine places the boundary between shades can exaggerate or hide a change. It should always be read alongside the numbers, never on its own.

Total deviation and pattern deviation

These two maps compare your results to what is expected.

Total deviation compares each point to an average person of your age. Sensitivity naturally declines with age, so the comparison is age-matched. Negative numbers mean you saw less well than expected.

Pattern deviation corrects for a general dimming across the whole field. This matters more than it sounds. A dense cataract dims the entire field, which can bury a small localised defect from glaucoma. Pattern deviation strips out the overall dimming so that focal loss becomes visible.

If your total deviation looks poor but your pattern deviation looks better, a general dimming, often a cataract, is likely part of the picture.

MD, PSD, VFI and GHT explained

You may also see SF (short-term fluctuation) and CPSD, which describe how consistently you responded when the same point was tested more than once.

What “24-2” and “10-2” mean

The number before the dash is how many degrees out from the centre were tested. A 24-2 covers the central 24 degrees; a 30-2 covers 30; a 10-2 concentrates on the central 10.

The “2” after the dash describes how the grid of test points sits relative to the horizontal and vertical midlines: the points straddle those lines rather than sitting on them, which makes it easier to see whether a defect respects the midline. That detail matters diagnostically, which is why the 2 pattern is standard.

A 10-2 is chosen when the central field needs closer study, for example in advanced glaucoma, or when monitoring certain medications known to affect the macula.

What Different Patterns of Field Loss Suggest

This table describes associations, not diagnoses. Patterns overlap, and no field map is interpreted without an examination, optic nerve imaging and your clinical history.

Can a Visual Field Test Be Wrong?

Yes, and the machine is built to notice.

A single field can be misleading for several reasons: fatigue, drifting fixation, an uncorrected prescription, a droopy eyelid casting a shadow, the rim of the trial lens creating a false edge, small pupils, or simple unfamiliarity with the test. This is exactly why the reliability indices exist: a test that does not hold together internally is flagged and repeated rather than acted on.

The learning effect is real and worth knowing about. First tests are often worse than later ones, purely through inexperience. A single poor first result is not treated as a diagnosis.

The corollary is worth stating plainly: because responses are cross-checked against your own natural blind spot and against lights that were never shown, a test that is not done honestly shows up as unreliable rather than as a convincing result.

Why One Visual Field Test Is Never Enough

If you have glaucoma, you will be called back for this test repeatedly, and it is reasonable to wonder why.

Field results vary between visits even when nothing has changed; that is the nature of a test built on human responses. Distinguishing a genuine downward trend from ordinary variation needs several data points. Published interpretation guidance holds that generally six or more visual field tests are needed to assess whether the disease is progressing, and that a new field should be compared against the whole earlier series rather than just the last one.

Glaucoma treatment aims to slow the rate of loss. You cannot measure a rate from one point. Each repeat test is another point on the line, which is why keeping the appointments matters even when your vision feels unchanged.

When to Contact Your Eye Specialist

Book an earlier appointment if you notice:

• Bumping into objects on one side, or missing steps and kerbs
• Difficulty with night driving or judging distances
• A patch of vision that seems missing or shadowed
• Sudden loss of part of your vision in one or both eyes
• Eye pain with redness, blurred vision and coloured haloes around lights

The last two need urgent attention. Sudden field loss can indicate a vascular event affecting the eye or the brain, and the combination of pain, redness and haloes can indicate an acute rise in eye pressure. Neither should wait for a scheduled review.

If you have diabetes, keep your retinal screening appointments alongside any field testing diabetic retinopathy affects a different part of the eye and needs its own examination.

Conclusion

The visual field test asks something slightly unnatural of you: stare at one point, notice things you are not looking at, and accept that many of the lights are meant to be invisible. Understood that way, it stops feeling like an exam you might fail.

What it produces is the only reliable record of how much vision you actually have across your whole field the part of your sight that glaucoma takes first and that no eye chart measures. A single test establishes a starting point. The value comes from the series.

If you have glaucoma, a family history of it, or you have been advised to have your optic nerve monitored and have not been back in a while, book a comprehensive eye examination. Field loss from glaucoma cannot be reversed, but its rate can usually be slowed and that depends entirely on catching the trend early.