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. Most younger people who wear spectacles have single vision lenses but as people get older, they gradually lose the ability to alter the focus of their eyes ('presbyopia'), and usually some time between the ages of 40 and 50, they develop a need for corrective lenses for both distance and near vision. They can use two pairs of single vision spectacles and 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 different types of multifocal lenses: bifocal 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 aAnother way of providing multiple corrections in one lens is to have a gradual change between the distance and near corrections. This type of lens is known as a progressive addition lens (also known as a "PAL" or "progressive" lens). This type of lens allows the wearer to focus on all distances, there being a gentle change from distance to intermediate to near focus as one looks through different parts of the lens. lower segment is used for reading and other close tasks such as sewing.
