Types of eyeglass lenses according to optical effect used
Prescription eyeglass lenses can also be divided according to the optical effect. According to the optical effect We know unifocal, bifocal, trifocal and multifocal eyeglass lenses.
Unifocal prescription lenses
Under unifocal eyeglass lenses we understand all spherical lenses, cylindrical lenses and lenses with aspherical surface and one focal point, or two focal points with the cylindrical lenses.
Spherical prescription lenses
They are the most commonly used type of eyeglass lenses for correction of rotary symmetrical (spherical) errors of refraction. The radius is constant on functional optical surface.
Cylindrical prescription lenses
They are used for correction of astigmatism. The way how they refract light beams is how they differ from spherical lenses. Parallel light beams are not focused in one point, but there are two focal points in different distances.
Aspherical prescription lenses
Eyeglasses should fulfill all demands on good vision but also satisfy requests and wishes of their wearers, e.g. look good and be the lightest possible. Aspherical eyeglass lenses fulfill al these demands with their aspherical surface. It is a surface with variable radius of curvature. So it is not a round surface, but rotary aspherical surface, which gets flatter towards the edge. This enables to manufacture a thin and light lens, which has very good displaying abilities on the periphery. Aspherical prescription eyeglass lenses play an important role with plus diopter and very high minus diopter.
Bifocal prescription lenses
Bifocal eyeglass lenses, or lenses with two focal points, have another “additional” lens connected with the “basic” one-focal point lens. The basic part is used for looking long-distance and the additional piece for looking short-distance. The additional part is either sealed or cut into the basic part. The shapes of the segments were changing with the development. Starting from flat boundary, through round piece for short distance, to a segment shaped like a lying letter D with slightly arched course of the upper part of the segment.
The biggest disadvantage of bifocal lenses is a big difference in the picture when changing the distance of looking. This type of lenses is in a recession.
Trifocal prescription lenses
Trifocal eyeglass lenses are used when the additional part for short distance is over two diopters and at the same time the drop of accommodative width is under 2 diopters. The difference between bifocal and trifocal eyeglass lenses is in inserting a middle piece for middle distance. This type of lenses never really got popular and is hardly used.
Multifocal prescription lenses
Multifocal (progressive) lenses, unlike bifocal lenses, enable a sharp vision in all distances and have a fluent transition from the short-distance piece to the long-distance piece. The dioptrical value changes continuously from the zone of long-distance vision, through a corridor (progressive canal) for middle-distance vision to the zone for reading. They are manufacture from one piece of optical material. It is the ideal type of lenses for active people with presbyopia.
The quality of the prescription eyeglass lenses comes not only from the material used, but also from the surfacing. Every good lens should have the anti reflective coating, which eliminates unpleasant reflections from the lens itself. With the plastic eyeglass prescription lenses it is desirable to harden them to prevent scratching, because plastic material is too soft. The hydrophobic adjustment is used for easier cleaning, crystal surface adjustment is used with the highest categories of lenses and these adjustments drive away the dirt particles because of extremely smooth surface. They also decrease the risk of contamination and trapping of dust particles, which can lead to scratching the lens and make lenses easier to clean.
If you choose prescription sunglasses or sports prescription eyeglasses, or eyeglasses with greater curvature, it is not a problem nowadays. Also, every high quality lens, colored or transparent contains a UV filter.
