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.
Figure 1-29 Conoid of Sturm, the geometric figure formed by the rays of light leaving an astigmatic lens. The circle of least confusion is the circular cross section of the conoid of Sturm that is halfway (in diopters, not linearly) between the 2 focal lines.
(Reproduced from Guyton DL et al., Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press; 1999.)
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.