ABO NOCE Practice Test

When calculating spherical equivalent, what is done with the cylinder power?

1. Ignore it completely

2. Double it and subtract from sphere

3. Half it and add it to the sphere power

4. Half it and subtract from the sphere power

The correct answer is: Half it and add it to the sphere power

When calculating the spherical equivalent, the approach involves considering both the sphere and cylinder powers. To find the spherical equivalent, the cylinder power is halved and then added to the sphere power. This method is rooted in the desire to transition from a prescription that includes astigmatism (which is represented by the cylinder power) to a simplified form that can be easier to interpret and use, especially for patients who may not need correction for the astigmatism. Halving the cylinder power accounts for the fact that the correction for astigmatism is typically not as intense as the spherical power and allows for a more balanced representation of the overall refractive error. This combined value is what is typically referred to as the spherical equivalent, which provides a more practical number for prescribers and for patients who may benefit from a less complex prescription.