Multifocal Lenses are characterised by a gradient of increasing lens power, added to the wearer’s correction for the other refractive errors. The gradient starts at the wearer’s distance prescription, at the top of the lens and reaches a maximum addition power, or the full reading addition, at the bottom of the lens.
Bifocals – which were invented by Ben Franklin – compensate for presbyopia by providing two different focusing powers in one lens. One area of the lens helps you focus on distant objects, the other for items that are close up. The problem with bifocals has always been that in reality we have the need to see clearly at many distances, not simply two distances.
Trifocals are eyeglasses where the lenses have 3 regions to correct for distance, intermediate (arm’s length), and near vision. They are mostly used by people with advanced presbyopia who have been prescribed 2 diopters or more of reading addition. The intermediate addition is normally half the reading addition.
Trifocals are becoming rarer as more people choose to wear progressive lenses.