Bifocal lenses are lenses in eyeglasses that combine different powers. One section of the lens is for viewing objects at a distance while the other is for viewing objects at close range. There are several types of bifocals lenses that vary in the types of lines that divide the lens sections as well as the proportion of the viewing sections.
The executive bifocal lens is divided into two parts by a line that is visible from either side of the lens. The top part of the lens, above the line, is for viewing things at a distance, while the bottom part of the lens, below the line, is for viewing things up close. For example, such lenses make it possible for someone to drive while looking through the top section, and have no trouble reading a map through the bottom section.
Flat Top Bifocal
The flat top bifocal lens is in the shape of a half moon and is sometimes referred to as a D rim. This lens is good for those who only view near objects on occasion as the lower region of the lens that is for near-point viewing, is smaller. The flat top bifocal is similar to the executive bifocal, but the executive bifocal is dividing more evenly between the upper and lower section of the lens while the flat top allows more lens for distance viewing than near-point viewing.
The progressive lens, also known as a no-line bifocal, is similar to the other bifocal lenses. The difference is that the line on the lens that divides the types of viewing isn't noticeable. While on the executive line you can obviously see the line that divides the upper section from the lower section, on the progressive lens, the change from the upper lens, which is for distance viewing, to the lower lens, for near-point viewing, is gradual.