Difference between Spherical Lens and Aspherical Lens
Generally speaking, spherical lenses are thick and the images through the lenses will be distorted. While, with the same prescription and index, aspherical lenses are thinner, lighter and more comfortable for wearers. Aspherical lenses have a wider vision and the images through the lenses are more real and natural. Wearers will not feel tired after wearing aspherical lenses for a long time.
PD is the distance between your two pupils in millimeters. Having a correct PD on your glasses ensures that you are looking through the ideal spot in your lenses. If there is no PD on your prescription, you can check it with your doctor or measure it yourself.
Sphere(SPH) refers to the refractive correction in the prescription. Minus(-) values are for nearsightedness, and Plus(+) values are for farsightedness. If "PL" or "Plano" is written for the either SPH on your prescription, then you should select "0.00".
CYL(Cylinder) & Axis
Cylinder(CYL) refers to astigmatism. It can be either positive(+) or negative(-). Axis is recorded as an angle in degrees, between 0°and 180°. Therefore, if there is a CYL value on your prescription, then there must be an Axis for it.
If "DS" or "SPH" is noted for the either CYL on your prescription, it means you have no astigmatism.
The number sometimes written as "NV" or "Reading Addition" refers to near vision and is used for multifocal lenses and readers.
Prism is power, measured in diopters, used to correct vision displacement, like double vision or an eye turn. Most eyeglass prescriptions do not have prism correction. A prescription with prism will have two values per eye: a Prism (power) and Base (direction).
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