Research Notes

The sag: A forgotten parameter in need of resurrection?

Wayne D.H. Gillan
African Vision and Eye Health | Vol 75, No 1 | a352 | DOI: | © 2016 Wayne D.H. Gillan | This work is licensed under CC Attribution 4.0
Submitted: 11 February 2016 | Published: 16 September 2016

About the author(s)

Wayne D.H. Gillan, Department of Optometry, University of Johannesburg, South Africa

Share this article

Bookmark and Share


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.


Sag; corneal height


Total abstract views: 3341
Total article views: 7199

Crossref Citations

No related citations found.