Astigmatism and the toric intraocular lens and other vertex distance effects.

An intraocular lens with a given cylindrical power will correct a variable amount of cylinder at the spectacle plane. This variability depends not only on the vertex distance but also on the degree of ametropia. The amount of spectacle cylinder corrected will be more for myopes and less for hyperopes. Anterior corneal cylinder (as determined by the keratometer) is a more reliable way of estimating the cylinder needed at the intraocular lens plane.