When determing the lens type, the patient's K-readings and spectacle refraction must be taken into account.
The basic principle is that when a rigid lens is placed on the eye, the tear layer under the lens (often called the tear lens or lacrimal lens) will correct (or create) an astigmatic error equal to the corneal toricity (amount of cylinder found with the keratometer). This assumes that the contact lens does not bend or flex on the eye.
A hydrogel lens will conform to the cornea with no lacrimal lens effect, therefore the spectacle cylinder will still be present with the hydrogel lens on the eye. An over-refraction will give the same cylinder as in the spectacles (taking into account any vertex distance correction).
