Lens designs The most commonly prescribed spectacle lenses are 'single vision' lenses. Single vision lenses have only one power in the lens and allow you to see clearly in either the distance or at near. They can use two pairs of single vision spectacles and to change them as necessary or, alternatively, one pair of multifocal lenses. As the name suggests multifocal lenses are lenses which have more than one power, allowing the wearer to focus at more than one distance. There are three main types of multifocal lenses, bifocal, trifocal and progressive.
Bifocal lenses have two distinct segments with different powers.
In most cases a top segment is used to see in the distance while a lower segment is used for reading and other close tasks such as sewing. Trifocal lenses are prescribed for people who have a need to focus on the mid range in addition to near and distance. For example, bank tellers may need to read forms passed to them, see the customer standing in front of them and be able to see across the bank. There is a distinct dividing line between the segments. Another way of providing multiple corrections in one lens is to have a gradual change between the distance and near corrections.