How to understand it
This is geometry, distinct from the SPH prescription field. Astigmatic prescriptions can involve toric surfaces as well.
An example
A spherical lens can be cut to fit a rectangular frame; the outer outline is not what “spherical” describes.
A common mix-up
Do not confuse spherical design with single vision or the absence of cylinder correction.
See the idea in a full guide →
Sources & verification
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