Identify a primary characteristic of an aspheric lens.

An aspheric lens is characterized by its variable curvature. This design feature allows the lens to have a gradually changing curvature from the center to the edges, rather than maintaining a consistent curvature throughout. This gradation can help reduce optical aberrations, improve visual acuity, and enhance comfort for the wearer. By having variable curvature, aspheric lenses can provide a more natural field of vision, which is particularly beneficial for individuals who require stronger prescriptions or have astigmatism.

In contrast, options like fixed curvature would indicate a lens with a uniform shape, which does not allow for the improvements in optical quality that aspheric designs can achieve. Thicker edges or progressive power distribution do not solely define aspheric lenses; they may pertain to different types of lenses or designs within the lens category. Thus, the essence of an aspheric lens lies in its ability to vary curvature to optimize vision and comfort for the wearer.