2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 1: Geometric Optics
Aberrations
The Spherical Equivalent
The spherical equivalent (SE) is the “average” power of a toric lens. For instance, a +2.00 +3.00 × 010 cylinder has a power of +2.00 D in the 10° meridian and +5.00 D in the 100° meridian. The average is +3.50 D, which is the SE power. If the lens is described in spherocylindrical notation, we calculate the SE as the sum of the sphere and half the cylinder power. For instance, a +2.00 −5.00 × 020 spherocylinder has the SE of −0.50 D.
We can use the SE to calculate the location of the circle of least confusion. For example, when the SE is +3.50 D and the object vergence is −1.00 D, the circle of least confusion is +2.50 D, or 40 cm to the right of the lens.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.