A Spherical Equivalent is a set of two numbers, one value for each eye, that gives you an estimate of your eyes' refractive error. It is not the most accurate number for most people as it does not include the astigmatism component. The Spherical Equivalent is calculated as follows:
The axis is removed
- The axis is not part of the calculation for spherical equivalent at all. It completely disappears.
The cylinder is divided by 2
- The cylinder power is only present in a particular direction. In other words, it is only present in half the lens. When attempting to combine the cylinder with the sphere, your doctor must take this into account by only taking half of the cylinder.
- Because cylinder powers only come in steps of -0.25, it is possible that when it is divided by 2, that the number does not end in a multiple of -0.25. That is where your eye doctor’s professional judgement comes into play to decide which closest multiple of -0.25 will be the most appropriate for you based on many different considerations.
The sphere and the 1/2 cylinder are combined
- The sphere and the cylinder are then added together to give you the equivalent sphere.
An example of this would be:
Theoretical Spherical equivalent: