Varifocal lenses. Multifocals. Progressive lenses. They are known by many names, and do many things.
The varifocal lens is a marvellous work of optical technology that allows for up to three prescriptions in a single lens. This means they can correct your near, intermediate, and distance vision, all without the need to change your specs.
This is a complex area of optics, and can be tricky to get your head around. This guide will give you a better understanding of the technical brilliance of varifocals.