Eyeglass lens types

Single-Vision, Full-time wear

For people whose distance prescription will be comfortable at all distances (far and near), doctors prescribe single-vision full-time eyeglasses. This is the most common prescription, particularly for those who are nearsighted. This type of prescription typically becomes less comfortable after the age of 40, when presbyopia begins and bifocals or progressives become the lens-type of choice.

Nearsighted people can sometimes still wear single-vision glasses long after 40 years old if the amount of nearsightedness they have is roughly equivalent to their reading prescription. Then they only need to take their distance-prescription glasses off for perfect up-close vision.

Single-Vision, Near-only

People whose accommodative (focusing) system isn't as well as others, either due to age (presbyopia) or other factors will require a prescription for up close. They are most commonly just called reading glasses or computer glasses.

They are used with people whose distance vision is fine (or they're wearing distance-only contact lenses), so they only need eyeglasses for up-close tasks.

Bifocals

bifocals are the eyeglass-types worn by people after they turn 40, when presbyopia typically begins. The top half of the glasses has the distance prescription. Then there is a line that separates that from the bottom portion that contains the reading prescription. So when the person using them looks all around they have good far-vision. People typically look below eye-level for reading, so the reading part (typically called the segment) is placed where it's comfortable on that person.

bifocals are available in various widths. 99% of the bifocals sold in the USA are called flat-top-28 bifocals. This just means that the bifocal has a flat horizontal line ("flat-top") that is 28mm wide. It is ample space for almost everyone without being to large to become unsightly. Bifocals also come in executive (full-width), 25mm and 35mm widths, though they are far less-commonly used.

Some people wear what are called no-line bifocals or progressives, which is discussed in the next section.

Progressive Lenses ("no-lines")

People over 40 wear either bifocals or progressives so they can enjoy good vision both far-away and up close. Though many are comfortable with it, frequently patients prefer the advantages of a bifocal prescription without the line on the glasses. These lenses are called progressive lenses, and are sometimes called referred to with many terms (no-lines; no-line bifocals; no-line trifocals; varifocals; Varilux®).

Progressive lenses are more pleasant-looking than traditional bifocals, simply because they look the same as a regular pair of single-vision eyeglasses. The top has the distance prescription, the bottom has the reading prescription, and in between is for the intermediate vision. The further you look down, the more power you get, and the closer you can see things. Instead of a sharp change from distance-to-near vision like a traditional bifocal, the change is gradual, or progressive, hence the name.

There is a slight disadvantage to progressives, though most consider it only minor. The side-portion of the lenses, where the intermediate vision is located, is slightly distorted. This can be overcome by simply pointing your nose at what you want to see at these distances. It usually takes a day or two to get used to, but after that it becomes second-nature. Better progressive lenses have wider intermediate zones, minimizing this effect.