A Spherical Equivalent is an estimate of your eyes’ refractive error, calculated independently for each eye. It is calculated by merging the spherical (nearsightedness or farsightedness) and cylindrical (astigmatism) components of your refractive error but is usually not accurate enough for ordering glasses that would provide the sharpest vision. An analogy would be converting a football-shaped surface where half of the surface is steeper than the rest into a completely spherical surface like a basketball. It is not an exact fit, but the idea is to get as close as possible.
Spherical equivalents are often used by eye doctors to prescribe contact lenses for patients with low astigmatism or patients who want colored contact lenses (very few colored contact lens brands are able to correct astigmatism), to reduce the astigmatism in an eyeglass prescription for patients who have trouble adapting, or to compare overall changes in prescriptions.
The Spherical Equivalent is calculated as follows:
Spherical Equivalent = Cylinder/2 + Sphere
The axis is removed
The axis is not part of the calculation for spherical equivalent at all. It completely disappears, because we are converting the surface into a sphere, so half of the surface is no longer steeper than the other.
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, the steeper half. When attempting to combine the cylinder with the sphere, your doctor must take this into account by only taking half of the cylinder.
- Because lens powers only come in steps of 0.25 Diopters, it is possible that when the cylinder is divided by 2, it 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 various factors.
The sphere and the 1/2 cylinder are combined
- The sphere and the cylinder are then added together to give you the equivalent sphere. Be sure to account for the plus or minus sign.
An example of this would be:
Theoretical Spherical equivalent:
|-0.50/2 + (-2.00) = -2.25
|-0.50/2 + (-2.50) = -2.75