What determines the power of a bifocal lens?

The power of a bifocal lens is primarily determined by the difference in the refractive power needed for distance vision and the refractive power required for near vision. This difference is measured at the back vertex of the lens, which allows for effective correction of both distance and near vision simultaneously.

In a bifocal lens, the upper part of the lens is designed for distance viewing, while the lower segment is optimized for near tasks, such as reading. The respective powers of these segments are tailored to the individual's visual needs, and the difference between these two powers is what establishes the overall functionality of the bifocal lens.

Other factors like lens thickness, curvature, and material type may influence the lens's weight, aesthetics, and how light passes through it but do not directly determine the lens power in terms of visual correction. The power is strictly a function of the optical characteristics necessary for achieving clear vision at different distances, making the difference between the back vertex powers for distance and near the crucial element in this context.