Cutting a portion of a lens with a convex or concave shape from a sphere is called spherical cheap eyeglasses near me lens. Due to the unavoidable occurrence of aberration (especially astigmatism and image bending aberration) in spherical lenses.

So, there are lenses that compensate for the shortcomings of spherical lenses, namely non-spherical lenses (usually referring to single-sided nonspherical lenses), which refer to rotating surfaces with the same non-circular cross-section on all meridians (such as parabolas).

Its biggest advantage is that there is no distortion of objects in the edge field of view, and the edges of the lens are thinner than ordinary spherical lenses. So, the price is relatively higher, and there are also double-sided aspherical lenses on the eyeglass mart, which make wearing them clearer and have better imaging effects. Of course, the price is also very expensive.

The Abbe number is the dispersion coefficient, an important indicator for measuring the clarity of lens imaging. Usually represented by the reciprocal of the dispersion coefficient, also known as the Abbe number.

The larger the Abbe number, the smaller the dispersion; conversely, the smaller the Abbe number, the greater the dispersion, and the poorer the clarity of its imaging. The Abbe number of a lens is related to its refractive index. The higher the refractive index of the material, the more severe the dispersion, that is, the lower the Abbe number.

So, in terms of this single indicator, the lower the refractive index, the better. However, when choosing discount prescription glasses, it is necessary to judge based on the comprehensive situation of one's vision, which is most important and cannot be mechanically limited to the Abbe number.