Astigmatism Test
Calibrate your screen with a bank card, step back to the distance shown, cover one eye and look at the center of the clock dial. If some lines look darker, sharper or blacker than others, click them. The test is a quick screening aid only; it cannot diagnose astigmatism.
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Not a medical test. This chart is a screening aid for curiosity only. It cannot diagnose astigmatism or any eye condition and does not replace an eye exam. See an optometrist or ophthalmologist for any vision concern, and urgently for sudden changes.
1 Calibrate your screen
Hold a bank card, debit card or driver's license (standard 85.6 × 54 mm) against the screen inside the outline. Move the slider until the outline is exactly as wide as the card.
2 Set up and look at the chart
3 Which lines look darker or sharper?
Click the lines on the chart or the buttons below. Choose more than one if several neighbouring lines look darker.
Your results
How to use the Astigmatism Test
- Calibrate the screen: hold a bank or ID card against the outline and drag the slider until the widths match, then save.
- Choose your viewing distance and stand or sit at exactly that distance. Wear your usual glasses or contacts if you want to check them.
- Cover your left eye, select Right eye and look at the center dot of the dial for a few seconds.
- Click any lines that look darker, blacker or sharper, or choose All lines look the same.
- Repeat for the left eye, then read the results and copy them if you plan to book an eye exam.
What astigmatism is and why the dial shows it
An ideal eye focuses light from every direction to a single sharp point. In astigmatism, the front surface of the eye (the cornea) or the lens is shaped more like the back of a spoon than a ball, so it bends light more strongly along one direction (meridian) than along the direction at right angles to it. The result is two focal lines instead of one focal point: lines that run in one orientation come into focus and look crisp and black, while lines at other angles are smeared and look grey.
The astigmatic dial (also called a clock dial or sunburst chart) has groups of identical lines radiating at 30-degree steps. Because every group has the same width and contrast, any difference you see comes from how your eye focuses each orientation. Eye care professionals still use the dial as a quick subjective check before fine-tuning a prescription.
How the chart is sized
After calibration, the page knows how many screen pixels make a millimetre. Each line is drawn with a width that covers 1.5 arcminutes of visual angle at your chosen distance, using width = distance × tan(1.5′). At 6 ft (1.83 m) that is about 0.80 mm; at 10 ft (3.05 m) about 1.33 mm. The gap between lines equals the line width. Fine lines like these make uneven focus easier to notice than thick ones.
| Darker lines (clock) | Line orientation | Rule-of-30 axis estimate |
|---|---|---|
| 12–6 | Vertical | 180° |
| 1–7 | Tilted 30° from vertical | 30° |
| 2–8 | Tilted 60° from vertical | 60° |
| 3–9 | Horizontal | 90° |
| 4–10 | Tilted 120° from vertical | 120° |
| 5–11 | Tilted 150° from vertical | 150° |
The rule of 30 multiplies the lower clock number of the darkest line by 30 to estimate the axis of the correcting minus cylinder in a prescription. It is a teaching shortcut; the real value comes from a professional refraction.
Tips for a fair result
- Distance matters: measure it with a tape measure rather than guessing, and keep your head level.
- Cover, don't press: pressing on the covered eye can blur it for minutes afterwards when you switch.
- Look at the center dot for a few seconds and let your focus settle before judging the lines.
- Check for screen effects: use the 15° rotation, and try tilting your head. Darkness caused by your eye turns with your head; darkness caused by the screen stays put.
- Bright, even lighting and full screen brightness give the most consistent contrast.
- See a professional for blurred or doubled vision, eye strain or headaches, and urgently for sudden vision changes, flashes or a curtain over your vision, whatever this chart shows.
Frequently asked questions
How does the clock dial astigmatism test work?
In astigmatism the cornea or lens bends light more strongly in one direction than in the direction at right angles to it. Lines that run along one orientation therefore focus sharply and look darker, while lines at other angles look grey or blurred. With normal focus, all lines of the dial look equally black.
Can an online test diagnose astigmatism?
No. This chart is a screening aid that can hint at uneven focus. Only an eye exam with refraction or keratometry by an optometrist or ophthalmologist can diagnose astigmatism and measure how much you have. See a professional for any vision concern, whatever this test shows.
Why do I need to calibrate my screen?
Screens have very different pixel densities, so a line drawn with a fixed number of pixels can be thick on one screen and hair-thin on another. Calibrating with a standard 85.6 mm card lets the page draw lines with a known physical width, which is then matched to your viewing distance.
What does it mean if all the lines look equally dark?
It suggests there is no significant uneven focus in that eye at this distance, which is the expected result for most people or for well-corrected glasses. Small amounts of astigmatism may not show on this chart, so it is not a guarantee.
What is the rule of 30?
Eye care professionals use it with the clock dial: take the lower clock number of the darkest line and multiply by 30 to estimate the axis of the correcting minus cylinder. Darker 3 to 9 lines give 90 degrees, 12 to 6 lines give 180 degrees. It is an estimate; your prescription comes from a full exam.
The horizontal and vertical lines look darker on every screen. Why?
Lines that line up with the pixel grid can render crisper than diagonal ones. Turn on the 15 degree rotation: if the darkness moves with the chart it is the screen; if a line at the same direction in space still looks darker, it is more likely your eye. Tilting your head is another check, because darkness caused by the eye turns with your head.
Should I do the test with my glasses on?
Do it without correction to check your eyes, and with your glasses or contacts to see whether they correct the problem well. If lines look uneven even with your current glasses, mention it at your next eye exam.
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