What Does Spherical Equivalent in Eyeglass Prescriptions Mean?

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Often times eye care professionals will talk about the ‘spherical equivalent’ in your eyeglass prescription. The spherical equivalent is a calculated measurement that’s included in the power of your lenses. This measurement is based on the shape of your cornea (the front of your eye) which can be oval or round shaped. 

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In the human eye, the cornea is responsible for most of the focusing power. Most of this power is in the spherical form and a lesser part is in the cylindrical form. Since most of the power of the eye is in this spherical form, eye doctors will talk about an estimate of the overall power of the eye as the ‘spherical equivalent’. The spherical equivalent is calculated by taking half of the cylinder power and adding it to the sphere power.

Why would we do this, you might wonder.  Well, it is important for those who have an  more oval shaped corneas (similar to a football), because they are prone to astigmatism that curve along the eye. The spherical equivalent helps determine how much curve is implemented into their corrective lenses. 

There are many terms that eye care professionals use for writing a prescription, so it can be confusing for patients. Hopefully this explanation of the spherical equivalent makes sense and explains more about how your eye works. 

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