Ortho-K typically works best for myopia (short-sightedness) between -1.00 to -6.00. The amount of astigmatism reduction depends on the type of astigmatism and amount. The upper limits for astigmatism reduction are 1.50 diopters of “with-the-rule” corneal astigmatism and 0.75 diopters of “against-the-rule” corneal astigmatism. Ortho-K will not have an effect on the internal astigmatism.