Astigmatism is a refractive error in which the eye's focusing power differs with direction. Detect its magnitude and axis by comparing directional focus; correct regular astigmatism with a suitably oriented cylindrical lens, adding a spherical lens when necessary.
For regular astigmatism, the two principal meridians of the eye are perpendicular and have different optical powers, commonly because the cornea has unequal curvatures. The crystalline lens can also contribute. A point object consequently does not form one sharp point image: the idealised paraxial optics gives two focal lines at different distances, perpendicular to one another, with an intervening region of blur. An ordinary spherical lens changes the focus in both meridians together and cannot by itself remove their difference. Irregular astigmatism, for example with a scarred cornea, lacks this simple pair of uniform principal powers.[ref]
Physiology of Astigmatism by the American Academy of Ophthalmology. Introduction and sections on regular and irregular astigmatism
Several complementary observations reveal it:
- With a distant point or suitable letter chart, an eye with astigmatism gives blur that cannot be completely removed with different spherical lenses. In a chart of equally dark radial lines, some directions can appear clearer or darker than others; unequal appearance suggests unequal directional focus, but does not alone supply a full prescription.
- A narrow stenopaic slit admits rays through a restricted meridian of the eye. Rotate the slit and determine the spherical lens giving best distant vision in each principal direction. A difference between the best powers demonstrates regular astigmatism. This is an especially natural optical explanation for an 1867 answer: it isolates directions rather than simply asking whether vision improves through a small round aperture.
- Modern objective retinoscopy observes the moving light reflex from the retina while trial lenses neutralise it in each meridian. Keratometry measures reflected images from the anterior cornea; corneal topography maps a larger area and helps detect irregularity. These measure different things: keratometry does not by itself include all contributions from the crystalline lens. Subjective refraction examination refines the prescription with trial lenses, often using a crossed-cylinder test.[ref]Assessment of visual function by the College of Optometrists (2025). Unit of Competence 7, objective retinoscopy and subjective refraction, including power and cylinder axis
The prescription must specify both the difference in optical power and its orientation. If the required correcting powers in the two principal directions are and , choose the cylinder axis along direction 1. In the ideal thin lens model:Here is the spherical correcting power, the cylindrical correcting power and the cylinder axis, all powers measured in dioptres. A cylindrical lens contributes no extra power along its axis and its full power perpendicular to that axis. For example, required corrections of and dioptres are supplied by a sphere of dioptre and a cylinder of dioptres, with its axis in the direction requiring the smaller negative correction. These are correcting powers, not the uncorrected eye's intrinsic powers; confusing the two reverses the intended correction.
Spectacles with this sphere-and-cylinder combination are the central historical remedy. Present options also include toric contact lenses, appropriate rigid contact lenses for some irregular cases, and selected refractive surgery. The suitability of these depends on the cause and condition of the cornea; a spectacle cylinder cannot generally cancel all irregularities of an irregular cornea.[ref]
Astigmatism by the National Eye Institute. Treatment options: spectacles, contact lenses and surgery; modern additions are not attributed to 1867
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