How Do Bifocal Lenses Work?

Correctable Eyesight

When a person has perfect vision, it is because the light entering the eye is focusing correctly on the retina. If a person has trouble seeing things at a distance, it is called myopia (nearsightedness) and is caused by the incoming light rays bending too much or too soon, causing the eye to focus in front of the retina. Conversely, if the light coming into the eye does not bend enough or bends too late the focus is behind the retina. This condition causes the person to have problems seeing things close up, and is called hyperopia (farsightedness). These are known as refractive sight errors, and when an individual has both conditions they may elect to wear bifocal lenses instead of a different pair of glasses for each type of sight need. Bifocals have two areas that bend the light to just the right place for each wearer and each sight distance.

Original Process

Bifocal lenses were invented by Benjamin Franklin when he tired of switching from one pair of glasses to another depending on whether he needed close or distance vision. Originally, bifocals were two different pieces of glass fitted together into a frame, each with its own prescription. Today, they are typically made of plastic and are one piece per lens.

Distance Sight

The upper portions of bifocal lenses enable the wearer to see clearly from a distance by looking through the top half of the lenses. Typically the upper lens portion is made specifically to correct the individual wearer's vision, but at times is made without vision correction if a person has a particular problem with close sight such as reading. In this case, the wearer utilizes only the lower portion of the lenses and sees with his normal vision through the top portion.

Close-Up Sight

According to the All about Eyeglasses website, the lower half of bifocals is made specifically for close-up vision, and can be one of four types: a semi-circle shaped segment, a round segment, a narrow rectangular shape or the bottom half of the entire lens. The type of lens prescribed depends on the sight correction needs of the individual patient as well as the primary use of the glasses, such as computer work or reading.

Difference in Lenses

While traditional bifocals have an obvious line across or around the lower field, the Glasses Crafter website states that progressive bifocal lenses have a blended lens field that is not discernible. In a progressive lens, the different vision prescription powers increase gradually from near to far and are more cosmetically acceptable. Not all bifocal wearers can adjust to progressive lenses, and those who do typically encounter a period of adjustment while they learn to utilize the transition field. Progressive bifocals are also known as lineless bifocals, progressive addition lenses, seamless bifocals and no-line bifocals.