Single vision lenses correct one distance. Presbyopia — the loss of near focus after about forty-five — needs more than one power in a lens. Bifocals add a visible segment; trifocals add an intermediate segment; progressive lenses change power smoothly from top to bottom with no line.
Progressive design is the main technical competition between lens makers. Conventional progressives are moulded; free-form (digital) progressives are cut point by point on a CNC generator from a semi-finished blank, allowing individualised designs based on the frame, fitting position and the wearer's habits. Premium free-form progressives from Essilor (Varilux), Zeiss, Hoya, Rodenstock and Shamir carry the highest margins in the trade.
Occupational or office lenses are progressives optimised for desk and screen distances. Short-corridor designs suit small frames. When buying, ask for the design name, the corridor length, the fitting height needed and whether the lens is free-form.