Excessive Tilt Affects Power and Astigmatism in Spectacle Lenses

Pantoscopic tilt and face form angles strongly affect the power profiles of spectacle lenses, according to a study published in Optometry and Vision Science. 

Researchers measured various tilt angles in 5 spherical lenses (all with a 1.56 index of refraction) consisting of the following powers: +4.0 D, +2.0 D, plano, -2.0 D and -4.0 D. They used a wavefront aberrometer to measure whole-surface power distribution, measuring the plus-powered lenses at tilt angles up to 20° and the minus-powered and plano lenses up to 15°. Investigators limited their analysis to a 40 mm area of each lens, and obtained contour plots of the spherical equivalent (M) and astigmatism (J). 

Before researchers applied tilt to any of the lenses, differences in M and J values between the lens center and 20 mm off axis were approximately +0.22, +0.17, -0.19, and -0.52 D, as well as +0.05, +0.03, +0.06, and +0.24 D for the +4.00, +2.00, -2.00, and -4.00 D lenses, respectively. Once they applied tilt, the greatest changes occurred in the lenses with highest dioptric power. After tilting the +4.00 D lens a total of 17.0°, M and J powers at the downward-tilted end were approximately +5.21 D and +0.59 D, respectively, compared with +4.30 D and +0.15 D, respectively, on the other side of the lens. A 15.1° tilt of the -4.00 lens yielded M and J values of -6.33 D and +1.44 D, respectively, at the inferior portion of the lens, compared with values of -3.88 D and +0.02, respectively, in the superior portion. 

Investigators recorded similar results in the +2.00 D and -2.00 D lenses with respect to M and J values, but with less power change. They noted almost no change in the plano lens. 

“The results show that negative lenses yield a more significant change in power than positive lenses.” according to the investigators. “The practical implications of these results are applicable not only in the conventional ophthalmic dispensing of spectacle lenses but also in the one-piece designed prescription sunglasses or sports glasses, especially for customers with myopic refractive problems who require negative lenses having a dual purpose of correcting refractive errors and filtering ultraviolet radiation.”

Study limitations include a failure to account for patient center of rotation measurements.

Reference

Huang C-Y. Measurement and analysis of whole-surface power distribution in single tilted spectacle lenses. Optom Vis Sci. 2022;99(2):177-185. doi:10.1097/OPX.0000000000001838