Multifocal eyeglass lenses contain two or more lens powers to help you see objects at all distances after you lose the ability to naturally change the focus of your eyes due to age (presbyopia).

Executive bifocal lens (also called the Franklin bifocal).

Bifocals contain two lens powers; trifocals have three. Progressive multifocal lenses gradually change in power from the top half of the lens to the bottom, and thus contain many lens powers.

Though multifocal lenses usually are prescribed for adults over age 40 to compensate for presbyopia, in some cases bifocals are prescribed for children and young adults who have eye teaming or focusing problems that cause eye strain when reading.

In these cases, the bottom portion of the bifocal lens reduces the amount of focusing effort required to see near objects clearly.

Also, some research suggests wearing bifocals or other multifocal lenses may help control myopia in some children by reducing the focusing demands of reading and near work.

Regardless of the reason you need a prescription for near-vision correction, bifocals all work in the same way. A small portion in the lower part of the lens contains the power required to correct your near vision. The rest of the lens usually is for your distance vision.

The lens segment (or "seg") devoted to near-vision correction can be one of several shapes:

• A half-moon — also called a flat-top, straight-top or D segment
• A round segment
• A narrow rectangular area, known as a ribbon segment
• The full bottom half of a bifocal lens, called the Franklin, Executive or E style

Generally, you look up and through the distance portion of the lens when focusing on points farther away, and you look down and through the bifocal segment of the lens when focusing on reading material or objects within 18 inches of your eyes.