Spherical Equivalent
Optics and Optometry
The single sphere that best replaces a sphero-cylinder, with the meridians it sits between.
How it is calculated
- Spherical equivalent = sphere + half the cylinder.
- It is the power that places the circle of least confusion on the retina — halfway between the two focal lines, which is why the cylinder is halved rather than dropped.
- The tool also reports the two principal meridian powers, since the spherical equivalent is exactly their average.
- It is what a spherical soft contact lens or a cycloplegic screening figure reduces a prescription to, and it is how refractive error is reported in almost every epidemiological study.
- It is not a substitute for the cylinder in a spectacle prescription: the astigmatic blur is still there, merely balanced around the retina rather than in front of or behind it.
References
- Standard ophthalmic optics — see any refraction text for the derivation from the conoid of Sturm.
About these tools
OcuLearning Clinical Tools is part of an educational platform. These calculators are provided for learning and reference, and are not medical advice, a diagnosis, or a treatment recommendation.