Spectacle lenses correct a patient's vision and the benefits that each lens design provides, important to the patient, is provided by its style. For example, a single vision (SV) lens can be used for far, near or computer distance vision but only has one power that is useful. For a +2.50 add presbyope, a typical flat top (FT) bifocal provides two powers, distance and near vision while a progressive (PAL) delivers an infinite number of powers i.e., clear vision at all distances, from far to near. The illustration below describes useful distances of clear vision in SV, FT and PALs.

Single vision, progressive and lined multifocal lenses are described by segment shape and size. Multifocals, lenses with more than one focus, are available in a variety of styles . Progressives are available in a variety of styles also, typically with multiple corridor lengths so that they can fit into smaller frames and still provide excellent far to near vision. As you see changing the corridor lengths results in changes to the sizes of the near and intermediate zones.