Bifocals began when Benjamin Franklin had the lenses of two different spectacles cut in half and wired into a single frame. As technology has advanced, more complex lens configurations were created, with multiple areas of magnification, called segments, cut into the same lens. Today a variety of lens designs offer a convenient way to improve vision that range from simple to sophisticated.
Also known as the "D" style mutli-focal lens, this type of lens has an a magnifying area in the lower part of the lens that resembles a letter D turned on its side, with the flat side up. This is among the easiest of multi-focal lenses to adapt to. It is also an available lens design and can be produced in any lens material. The flat top of the lens provides a definite transition between reading and distance vision.
Also referred to as a round lens, the magnifying area in the lower part of the lens is a complete circle. The lens can restrict viewing up top, due to the round shape. There is less significant transition between reading and distance vision.
Curved Top Lenses
Also known as the C-style, the magnification area is similar to the "D" style lens. However, the top of the area is slightly curved upward. The rounded corners on the segment are often marketed as enhancing peripheral vision.
This lens most closely resembles the original bifocal lenses invented by Benjamin Franklin and are also known as executive, dual and full-segment lenses.
This lens is often used as an occupational lens for those who do a lot of reading. Because the magnification area is so large, this style of lens offers the widest reading range of any bifocal.
The ultex lens, also called the baseball segment, is similar to the executive lens, but with a curved rather than a straight transition. The wide segment makes ultex lenses an excellent choice for an occupational lens that offers a wider reading range.
Blended bifocal lenses are also referred to as the invisible or seamless bifocal. This lens is designed to conceal the transition from the magnification segment to the distance segment. This is accomplished by placing a blended range of 1mm to 2mm between the segments. This range can distort perspective at times that some may not adapt to easily.
Vocational lenses have multiple magnifying segments in each lens above and below the midline of the lens. Double D, double Franklin and double-circle lenses are common configurations.
Trifocals incorporate three different focal lengths, the primary prescription and the top and bottom halves of the segment, each at different magnifications.
Specialty lenses can combine the benefits of different lens configurations to best improve the vision of the user.
These lenses separate the magnification powers into three progressive fields. Intermediate viewing is at the top of the lens, while near vision is at the bottom, with distance in the middle. This lens can help reduce the stress experienced when viewing computer monitors for long periods of time.
Progressive lenses are true multi-focal lenses that more closely mimic the natural vision. The top portion of the lens is for distance vision. The center of the lens provides intermediate viewing, and the bottom allows for close reading. The transitions from one segment to another are smooth and seamless.