Often when a patient is wearing a contact lens there is some astigmatism that is not corrected. The amount of cylindrical correction required to correct this remaining astigmatism is called residual astigmatism.
Residual astigmatism occurs with spherical rigid lenses when the corneal toricity (cylinder) does not match the spectacle cylinder (corrected for vertex distance).
With spherical hydrogel lenses the residual astigmatism usually equals the spectacle cylinder (corrected for vertex distance).
If this residual astigmatism is less than 1.00 D. it is often not corrected.
