spherical power

When no correction is needed, the spherical power will sometimes be written as 0.00 and sometimes as plano (pl.)

That means the spherical power describes the most divergent meridian and the cylindrical component describes the most convergent.

By changing these lenses, the examiner is able to determine the spherical power, cylindrical power, and cylindrical axis necessary to correct a person's refractive error.

This is specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the spherical power.

For lenses that correct for astigmatism as well, three numbers are given: one for the spherical power, one for the cylindrical power, and one for the angle of orientation of the astigmatism.