Spectacle Lenses- Types
Broadly there are three types of spectacle lenses- single vision, bifocals and varifocals.
Single vision lenses are usually all that is required until the patient reaches middle forties, when they usually begin to find that their general purpose lenses are still fine for far vision, but that they are having difficulty adjusting for near work. This condition, called Presbyopia, usually means that the patient will require one correction for far sight (TV and driving), and another different strength for near. That means they either need to regularly swap from one pair to the other, which may be annoying if trying to keep an eye on the television and the newspaper at the same time, or consider another option. As previously mentioned there are various options to improve the appearance and performance of these lenses- Hi Index or Aspherics.
Bifocals were originally two single vision lenses- one for far, one for near which were split and glazed into the same frame, the top of the lens for distance and the lower part for near. These days bifocals are somewhat better made, but follow the same principal of having one area of the lens for far vision and a different part (usually a smaller segment) lower down for reading. Depending on the individual requirements, the near vision part can be set at any suitable height, it can be round, or flat or curved or all the way across the lens. The advantages are obvious- no more swapping glasses on and off, but so are the disadvantages- you can only see near things through the near segment, and farther things would be blurred through it. Older patients may also note that middle vision distances such as shelves in shops or a computer screen may become difficult- too close to see through the far part, but too far away to see clearly through the reading zone. Many are put off by the visibility of the reading portion of the lens, while others feel that they may be constantly be tripping over things. Safety is usually not a problem- people rarely look where they are placing their feet, and if necessary it means just dropping the head a little more to see over the top of the reading part of the lens.
Trifocals, as their name suggests have three focal strengths instead of the two in a bifocal. Above the near portion there is a smaller intermediate segment, usually set at a half to two thirds of the reading power, which allows the user to see distances at approximately arm’s length- the distances such as the dashboard of a car or a computer screen. The trifocal wearer will not come across the problem of having a distance that cannot be seen through either the far or near portion of the lens. The disadvantages of trifocals may be the cosmetic- there are two lines instead of one, and because the near portion needs to be in approximately the same position as that in a bifocal lens, the intermediate part has to be higher up, and therefore more “in the way”. Rarely are trifocals prescribed any more- their rarity makes them more expensive (probably more expensive than varifocal lenses), and except for specific requirements, most people would find the varifocal cosmetically better, and more natural than a trifocal.
Varifocals, also called progressive addition lenses are the most modern lenses available to correct for presbyobia. Instead of there being a near vision segment, the lens gradually changes in power as you look down the lens- far vision at the top, near at the bottom. Importantly because of this smooth transition, there is no sudden change as with a bifocal, but it also means that there is a middle vision distance, which is useful for viewing things farther away than you might hold it to look at- the supermarket shelves, shop windows, and the car dashboard will all be clearly visible through this portion of the lens! The advantages over bifocals are the cosmetic improvement and the increased range of near vision, but like a bifocal the varifocal is a compromise between far and near sight- however it is a compromise which most people find suits their requirements extremely well.