Both bifocal and varifocal lenses allow the wearer to focus through different lens prescriptions without changing eyewear.
Presbyopia is a condition that usually affects people over 40. It makes it difficult to see up close, such as while reading or sewing. Nearsighted people who develop presbyopia need two sets of prescription lenses: a pair for seeing long distances, and a pair for reading and other near-distance activities.
Bifocal lenses allow for one set of eyewear that corrects both nearsightedness and presbyopia. The lenses contain a prescription for each condition. The wearer can look through the top section of the lens to see far away and the bottom section while reading. A line is visible in the lens where the prescription changes.
Types of Bifocals
The section of lens used for close-range vision can be half-moon-shaped or D-shaped, round, or narrow and rectangular (called a ribbon segment), or make up the entire bottom half of the lens.
Varifocal lenses also combine distance and reading prescriptions into one lens, but the transition between prescriptions is seamless, meaning there is no line in the lenses. They also correct blurry vision in the intermediate range.
Advantages and Disadvantages
Unlike bifocals, varifocal lenses do not have a seam where the two prescriptions meet. They also allow for correction of mid-range or arm's-length vision. However, due to the way the lenses are made, varifocals create some distortion.