Corrective Lenses - Elements
Corrective lenses are typically prescribed by an optometrist. The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter-diopter steps (0.25 D) because most people cannot generally distinguish between smaller increments (ex. eighth-diopter steps / 0.125 D).
Components of a sphero-cylindrical correction.
Each power specification includes a spherical correction in diopters. Convergent powers are positive (ex. +4.00 D) and condense light to correct for farsightedness (hyperopia) or allow the patient to read more comfortably (see presbyopia and binocular vision disorders). Divergent powers are negative (ex. -3.75 D) and spread out light to correct for nearsightedness (myopia). If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.
If a patient has an astigmatism, the patient needs two different correction powers in two different meridians (horizontally and vertically for example). This is specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the sphere power. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence or most divergence.
Ophthalmologists record in "plus cylinder notation" where the cylinder power is a number of diopters more convergent than the sphere power. That means the sphere power describes the most divergent meridian and the cylinder component describes the most convergent. Optometrists use "minus cylinder notation" where the cylinder power is a number of diopters more divergent than the sphere component. Thus the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. (There is no difference in these forms of notation. They arise from the nature of the two professions and are easily converted between by people accustomed to working with sphero-cylindrical prescriptions. They are simply two ways to specify the same thing.)
The axis defines the location of the sphere and cylinder powers. The sphere is almost always 90° from the cylinder. (This is regular astigmatism, which is by far more common than irregular astigmatism where separations are other than 90°). Vertical is the 90th meridian and horizontal is both zero and the 180th meridians. The axis is the meridian 90° away from the cylinder power. To rationalize this, think of a soft drink can, or other cylindrical object. Note how the curvature is 90° away from its axis (the line it would spin around if you were to roll it).
•Since the cylinder and sphere powers almost always separated by 90°, the axis is also the location of the sphere component. If the lens is spherical (there is no cylinder component) then there is no need for an axis. A prescription like this is written with D.S. (diopters sphere) after the spherical power (ex. -3.00 D.S.).
So, a prescription of -1.00 +0.25 x 180 describes a lens that has a horizontal power of -1.00 D and a vertical power of -0.75. (The same prescription written in minus cylinder notation: -0.75 - 0.25 x 090)
 Other considerations
Single vision lenses correct for only distance or near vision. Patients with presbyopia or other disorders of accommodation often benefit from corrections for both distance and near vision (see Lens Types below). Infrequently, prism and base curve values may also be specified to correct for binocular vision disorders.