Hypermetropia and Astigmatism

This lesson completes "The Eye" by covering its final two refractive defects. Hypermetropia is myopia's mirror image — too little power rather than too much — and is corrected in an exactly analogous way. Astigmatism is a genuinely different kind of fault, one of shape rather than simple strength, and needs a correspondingly different correction.

Hypermetropia: too little power

Hypermetropia (also called hyperopia, or long-sightedness) is the complementary defect to myopia. Here the eye's total refracting power is too weak for the length of the eyeball — or equivalently, the eyeball is too short for the eye's own power. Light from a near object, which needs a relatively large amount of power to bring it to a focus, would come to a focus behind the retina if it could — but of course the retina intercepts the light before it gets there, so what actually forms is a blurred image. Distant objects, needing much less power to focus, are usually still seen clearly, since even the eye's weaker-than-normal power is often just about sufficient for light that's already arriving nearly parallel.

The near point

The near point of an eye is the closest distance at which an object can still be brought into sharp focus. For a normal eye, the near point is a fairly close, well-defined distance — conventionally taken as a standard reference distance for calculation purposes, since it varies somewhat between individuals and with age. For a hypermetropic eye, the near point is pushed further away than this normal reference distance, so objects that a normal eye could focus on comfortably up close simply cannot be brought into focus at all by the hypermetropic eye — they remain blurred no matter how much the eye tries to accommodate.

Correcting hypermetropia with a converging lens

The correction for hypermetropia is a converging (convex) lens placed in front of the eye, adding the extra power the eye itself is lacking. The lens is chosen so that an object placed at the normal reference near point — closer than the person can actually focus on unaided — is imaged by the corrective lens into a virtual image formed out at the person's own, more distant, actual near point, a distance their eye genuinely can focus on. The eye's own optical system then focuses this virtual image onto the retina as normal.

Astigmatism: a fault of shape, not just strength

Astigmatism is a different kind of refractive defect entirely — not simply "extra" myopia or hypermetropia, but a fault in the shape of the refracting surface. In an astigmatic eye, the cornea (or, less commonly, the lens) is not perfectly spherical; instead it is curved more sharply in one direction than another — for example, more steeply curved vertically than horizontally. Because the amount of refraction at a curved surface depends on how sharply it curves, this means the eye has different focusing power along different orientations, or meridians. Light entering along the more steeply curved meridian is focused at one distance, while light entering along the flatter meridian is focused at a different distance.

The practical effect is that a single object cannot be brought entirely into focus at once: features aligned with one orientation may appear sharp while features aligned with the perpendicular orientation appear blurred or stretched, rather than the whole image being uniformly out of focus as with simple myopia or hypermetropia.

Reading an astigmatic prescription: sph / cyl / axis

Because astigmatism requires correcting power that varies with orientation, on top of whatever ordinary (spherical) short- or long-sightedness a person also has, a spectacle prescription for an astigmatic eye is written with three separate values:

• Sph (sphere). The ordinary spherical power needed to correct any accompanying myopia or hypermetropia — a plain diverging or converging power applied equally in every direction, exactly as calculated in the earlier worked examples in this lesson and the previous one.
• Cyl (cylinder). The additional power needed specifically to correct the astigmatism itself. This power is delivered by a cylindrical lens component — a lens shaped so that it has optical power only along one particular direction, and no power at all along the direction perpendicular to it, unlike an ordinary spherical lens which has the same power in every direction.
• Axis. An angle, measured in degrees, specifying the orientation along which the cylindrical correction should be aligned, so that it matches the specific direction of the eye's own irregular curvature.