Axis Finder
Astigmatic clock dial with rule-of-30 axis readout.
For educational purposes only. A teaching and screening aid, not a medical device; it does not replace a standard clinical examination.
How to use
- Test one eye at a time with the other occluded, through the best sphere found so far.
- Fog the eye: add plus sphere until acuity is blurred to roughly 20/40–20/50, so that both focal lines lie in front of the retina. Without fogging, the dial gives unreliable answers.
- Press Test patient and ask: Do all the lines look equally dark and sharp? If not, which line is the darkest or sharpest? Tap that line on the dial (in Test patient only small dots outside the dial mark the choice, so the dial itself is never altered).
- If two neighbouring lines look equally dark, tap both: the axis is half-way between them (for example 15°). If every line looks the same, tap All lines look equal.
- Then add minus cylinder at the axis shown, in small steps, until all the lines look equal, reducing the fog as needed.
How it is scored
- The darkest line runs parallel to the focal line nearest the retina, and the minus cylinder that corrects it is placed at 90° to it. Because the examiner faces the patient, this reduces to the rule of 30: multiply the lower clock number of the darkest line by 30. The 1–7 line gives axis 030, the 12–6 line gives 180.
- Axes are reported in standard notation, 1 to 180 degrees anticlockwise as seen by the examiner, with horizontal written as 180 rather than 0. The plus-cylinder axis is the minus-cylinder axis plus 90°.
Limitations
- The dial finds the axis only. It does not measure the cylinder power, which is found by adding cylinder until the lines equalise, or by Jackson cross-cylinder.
- It relies on fogging and on the patient’s judgement of “darkest”; it is less precise than the Jackson cross-cylinder, especially for low cylinders.
- The dial resolves 30°, or 15° when two neighbouring lines are equally dark. Refine the axis with the Jackson cross-cylinder.
References
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation — clinical refraction, the astigmatic dial technique.
- Benjamin WJ, ed. Borish’s Clinical Refraction. 2nd ed. Butterworth-Heinemann; 2006 — subjective refraction, astigmatic dials and the fan-and-block technique.
For educational purposes only. The Clinical Exam tools are part of an educational platform. They are screening and teaching aids, not certified medical devices: a result depends on the screen (letter sizes are estimated for the type of device), the lighting and the testing distance. They are not a diagnosis, and any abnormal or doubtful result should be confirmed with a standard clinical test.