Standard contact lenses for the correction of nearsightedness (myopia) and farsightedness (hypermetropia) have a spherical design, that is, they look like part of a sphere (ball). But there is also an aspherical design that provides even sharper and more accurate vision.
They have one or both surfaces aspherical. In conventional spherical lenses, distortions (spherical aberrations) regularly occur, which interfere with a clear perception of the picture. They appear when a person looks to the side, and not directly, that is, with peripheral vision. The fact is that the peripheral zone of the lens refracts parallel light rays falling on it more strongly than its central zone. Therefore, peripheral light rays focus closer or farther on the retina than rays passing through the center. So there is a blur of the picture, a fuzzy image. Especially noticeable are the aberrations that occur in low light, at night.
This problem is solved with the help of an aspherical lens design, when the curvature gradually changes from the center to the periphery, which ensures that the light rays of different zones are focused at one point. Thus, the center has an almost spherical shape, and towards the edge the curvature changes, as a result, the surface has the shape of an ellipse. This improves image contrast and minimizes distortion. In most cases, the front surface has this design, but there are lenses with the back or even both surfaces of the aspherical design.