How to use the astigmatism test
- Calibrate your screen. Hold a bank card or ID card against the blue card on screen and resize it until both match, then press Save calibration. The calibration is shared with the online eye test and the Amsler grid test, so you only do it once per screen.
- Choose your viewing distance. Pick Arm's length for a phone or laptop, or 3 m (10 ft) for a large monitor or TV. The line width is calculated for that distance.
- Get ready. Keep your usual glasses or contact lenses on, sit upright in good light and cover one eye with your palm. Choose Both eyes or a single eye and press Start the test.
- Look at the clock dial. Keep your gaze on the centre and answer Do all lines look equally dark and sharp? If some look darker, tap that line group on the dial or choose its clock hours, for example 1–7.
- Check the second chart (optional). A star of black and white wedges helps to confirm the direction. Use Turn 15° or Slow spin: a dark direction that stays in place while the star turns comes from your eye; one that turns with the star comes from the screen.
- Switch eyes and read the result. The result for each eye is saved in this browser so you can compare it with a later test. Clear saved result deletes it.
What is astigmatism?
Astigmatism is a common refractive error in which the front surface of the eye (the cornea) or the lens inside it is not curved evenly. The surface is shaped a little more like a rugby ball than a football, curved more steeply in one direction than in the direction at right angles to it. Light passing through the two directions comes to a focus at different distances, so the eye cannot bring everything into sharp focus at once.
Small amounts of astigmatism are very common and often cause no trouble at all. Larger amounts can cause blurred or slightly stretched vision at all distances, eye strain, headaches after screen work or reading, squinting, and glare or streaks around lights at night. Astigmatism often occurs together with short-sightedness (myopia) or long-sightedness (hyperopia).
How the clock dial test works
The clock dial, also called an astigmatic fan or sunburst chart, is one of the oldest subjective tests for astigmatism. It has groups of identical lines radiating from the centre like the hours on a clock. Every group has the same thickness and contrast, so an eye that focuses evenly in all directions sees them as equally black.
An astigmatic eye focuses lines in one orientation more sharply than lines at right angles to it. Those lines look blacker and crisper, while the lines in the perpendicular direction look grey, fuzzy or doubled. Noting which lines stand out gives a rough idea of the direction of the astigmatism, which is why the test records the clock hours (for example 2–8) and the approximate angle of that line, measured anticlockwise from the horizontal 3–9 line as you see it.
Please do not compare that angle with the axis on a glasses prescription. Eye-care professionals use a specific convention for the cylinder axis, and the axis written on a prescription is not simply the angle of the darkest line. This page also never estimates the amount of astigmatism: that needs a professional refraction.
Why the second chart and the rotation help
Screens are made of square pixels, so horizontal and vertical lines are drawn perfectly crisp while diagonal lines are smoothed and can look a touch lighter. Reflections, a tilted head or a smudge on the screen can also make one direction stand out. The star chart contains edges in every direction. If a darker or sharper bow-tie shape appears and stays pointing the same way while the star turns, the effect is coming from your eye. If the darker area turns along with the star, it is a property of the screen or the drawing, not of your vision.
How to read your result
| What you saw | What it may mean |
|---|---|
| All lines looked equally dark and sharp | No sign of significant astigmatism in this screening. Keep having regular eye exams. |
| Some lines looked darker, in a similar direction on both charts | This can be a sign of astigmatism. An eye exam can measure it and tell you whether correction would help. |
| Darker lines that changed between charts or moved with the rotation | An inconsistent result, often caused by the screen, focus or tired eyes. Repeat the test later and mention any symptoms to an optometrist. |
| All lines looked blurred | Possibly another refractive error such as short-sightedness at this distance. Try the online eye test and book an exam. |
Limits of an online astigmatism test
This is a screening for curiosity and awareness, not an eye examination or a diagnosis. Its result depends on the calibration, the viewing distance, the screen and the light in the room. Uncorrected short-sightedness can blur every line, a dry or uneven tear film changes from blink to blink, and very small amounts of astigmatism are often invisible on a dial. Only an optometrist or ophthalmologist can measure astigmatism with keratometry, retinoscopy and a subjective refraction, and check the health of your eyes at the same time. Regular eye exams are recommended for everyone, including people with no symptoms.
Seek urgent medical care if you notice sudden vision loss, a curtain or shadow over part of your vision, flashes of light with many new floaters, sudden distortion or a painful red eye. These are not signs of ordinary astigmatism.
Troubleshooting
- The dial is too small or a size warning appears: recalibrate, choose a shorter distance such as arm's length, or use a larger screen.
- 12–6 and 3–9 always look darker: this is often the pixel grid. Turn the star chart and see whether the dark area moves with it. A dead pixel and screen test can also reveal a dirty or damaged screen.
- The result changes each time: blink a few times, rest your eyes, keep your head upright and test in steady light.
Privacy: the test runs entirely in your browser. Your answers, your last result and the screen calibration are stored only in this browser's local storage and are never sent to a server. More checks for your eyes and ears, such as the colour blind test, are collected on the health tests page.
Frequently asked questions
What is astigmatism?
Astigmatism is a common focusing error caused by a cornea or lens that is curved more steeply in one direction than the other. It makes vision blurred or slightly stretched at all distances and can cause eye strain and headaches. It is not a disease and is usually easy to correct.
Can an online test detect astigmatism?
An online clock dial can show signs that suggest astigmatism, such as some lines looking clearly darker than others, but it cannot diagnose it or measure how much you have. Small amounts are often missed and screen effects can cause false alarms. An eye exam is the only reliable way to find out.
What does it mean if one line looks darker?
If one group of lines on the clock dial looks darker or sharper than the lines at right angles to it, your eye may be focusing that direction better than the other, which is typical of astigmatism. Check that the effect does not move when the chart turns, and ask an optometrist to measure it.
Can astigmatism get worse?
Astigmatism can change over the years, which is one reason regular eye exams are recommended. A steadily increasing amount, especially in teenagers and young adults, should be checked by an eye-care professional because it can occasionally be caused by a corneal condition such as keratoconus.
Can glasses or contact lenses correct astigmatism?
Yes. Glasses with a cylindrical (toric) correction and toric soft or rigid contact lenses correct most astigmatism, and laser eye surgery or toric lens implants are options for some adults. An optometrist or ophthalmologist can advise which suits you.
Should I take the test with my glasses on?
Take it the way you normally see at that distance. With your usual glasses or contacts on, the test checks whether your current correction leaves some astigmatism uncorrected. Without them, it shows how your eyes see on their own.
Updated