How are corrective lenses made?

The lab technician selects a lens blank that has the correct segment (called an add) and a base curve close to the prescribed power. Then to make the power match the prescription exactly, another curve is ground on the back of the lens blank. In most labs, the equipment is designed to grind minus curves, so a strong, plus lens blank is usually selected. If the base curve is too strong, then a minus curve is ground in the back of the lens which reduces the total power of the lens. Advances in automation are rapidly changing how lenses are made. For example, the vast majority of labs now use computers to determine curve parameters and lens choice.