Explanation of the Prescription for Astigmatism
The spectacle or contact lens that is used for the correction of astigmatism is called a toric lens. Unlike the case of a spherical lens where there is one power (i.e. the same degree of curving) in all meridians, a toric lens has two powers that are at right angles to each other, corresponding to the flattest and steepest meridians of the lens. An astigmatic cornea is similarly shaped and thus a toric lens that is appropriately positioned (with respect to its rotation and angle of the flattest meridian to the horizontal) can be used to correct the eye's astigmatism. (The flattest meridian of the correcting lens would need to be placed along the steepest meridian of the imaginary toric lens that is effectively produced by the combination of an astigmatic cornea and human lens in the eye.)
Thus, from a prescription for a toric lens, the optician must be able to derive the two principal powers of the lens and the angle of the flattest or steepest meridian. The usual notation today (termed as "minus cylinder " notation) specifies three variables. (1) The highest positive power of any meridian of the lens. (2) The difference in power compared to the least positive power meridian, with a minus sign (-) in front of it. (This value is called the cylinder of the lens and denotes the degree of astigmatism.) (3) The axis of the cylinder (denoted in angular degrees to the horizontal).
As a cylindrical lens on its own is flat in one direction and curved in the other direction, the only meridian that has power is the curved one. The axis of a cylindrical lens is defined as being at right angles to the power meridian of the cylinder, or in the same direction as the meridian with no power. In a lens that that is designed to correct for spherical refractive error as well as astigmatism, i.e. a toric lens, the axis of a minus cylinder works out to be in the same direction as the meridian of greatest positive power.
The following is an example of a prescription that is in minus cylinder
form: +2.75 / -1.25 5 10. The greatest positive power
of the lens is +2.75; the cylinder power (power difference in the
lens) is -1.25 (making the weakest power meridian =+1.50); the axis
of the cylinder (meridian of greatest positive power of the lens)
is 10 degrees from the horizontal. The cylinder power of the correcting
toric lens is related to the degree of astigmatism of an eye. The
axis of the toric lens is usually indicative of the angle of the
flattest meridian of the cornea, which is the principal determinant
of astigmatism in the eye.