The advent of large-diameter rigid contact lenses (semi-scleral lenses) has necessitated renewed understanding of the concept of ‘the sag’ of a contact lens and/or cornea. This article reviews the concept of ‘the sag’ and how to calculate the sag for spherical and aspherical surfaces. Reviews of empirical studies investigating the sag of the eye are presented. Data for normal and keratoconic corneas for sags as measured with an Oculus Keratograph are also included. How to include empirical measurements obtained in practice, in the determination of a first-trial semi-scleral lens, is suggested.
The sag: A forgotten parameter in need of resurrection?