What do the often-used terms “spherical” and “aspheric” mean?

We often hear the terms "spherical design" and "aspheric design" used when selecting spectacle lenses. What do these terms mean?

 Spherical Design Lens

This term refers to ordinary concave or convex lens with a curve that looks as if it has been cut out of a sphere. One of the disadvantages of this kind of lens is that, unlike aspheric design lenses, increased power results in a thick and heavy lens.

 Aspheric Design Lens

This type of lens features an aspherically-designed front surface curvature.Compared with the spherical design lens, this design makes it possible to produce a thin, lightweight lens with less peripheral blurring.

Clarity: This is a method of evaluating lenses defined to compare how clearly the contours of objects or small characters can be seen when looking through a lens.