The principle is that the corrected luminosity is determined by the average value in a certain visual range. Beyond this photometric range, the pupil size can be adjusted through the ciliary muscles of the eyes to achieve focusing. The advantage of multifocal lenses is that the change of luminosity is corrected with the difference of sight distance, and the change process is a curve process, thus ensuring the correction of vision. This can reduce the frequency of pupil changes, thus reducing the sitting fatigue of ciliary muscles. This kind of multi-curvature multifocal lens is called fatigue lens.

It should be noted that the matching of fatigue lenses requires very professional optometry and optical glasses assembly technology, which has certain restrictions on the types of lens frames. The vertical length of the lens frame is also determined according to the channel length of the focal point.