Do you remember when you went to get kids eyeglasses online, did you choose a spherical or nonspherical lens (in fact, the vast majority of lenses on the eyeglass mart are nonspherical, so there's nothing special about them).

Spherical lenses, as the name suggests, have extremely low dispersion in restoring the central view, while the distortion of the edge view is significant and has a wider range, resulting in less-than-ideal comfort and field of view. Simply put, a sphere is, for example, if you take a glass ball and cut it off from a certain piece, the curvature is the lens of the sphere.

Regarding spherical lenses, I wonder if you have studied geography. Do you remember the unit position of the map projection? The projection farther away from the center of the sphere will deform more severely.

According to the above principle, if we use spherical lenses for our cheapest eyeglasses, the deformation is too severe, which can cause our eyes to see clearly in the middle. However, once we move away from the middle, what we see will become blurry due to severe deformation.

Nonspherical lenses greatly reduce the range of edge distortion, improve image restoration, broaden the field of view, and provide higher comfort. A simple understanding is that the front and back of a lens have different curvatures, which can be imagined as a combination of quadratic curves. So, it's not a shape that can be cut with a ball.