Theoretical aspects of concentric varifocal lenses.

The first-order design characteristics of a concentric varifocal lens consisting of a circular reading portion and an outer distance portion, each of contrast power and separated by an annular zone where the sagittal power decreases linearly with zonal radius, are analysed. It is shown that, while such a lens can either be constructed in homogeneous material of varying thickness (the Beach lens) or with a cylindrical gradient of refractive index and a constant lens thickness, substantial amounts of unwanted astigmatism in the transition zone are intrinsic to the design and to other forms of concentric varifocal. These theoretical results are compared with available practical measurements on Beach lenses.