A corrective eyeglass lens is one that is used in place of, or in addition to, the lens of the eye in order to increase visual acuity.
Eyeglasses are often chosen over contact lenses for a number of reasons.
There are many types of lenses from which to choose, as well as their coatings and treatments. However, the first to be chosen is often the frame.
Eyeglass frames vary considerably according to shape, colors, and materials. Some lens types will not work with certain frames. For example, if a prescription requires thick lenses, a thin wire frame may not be able to support them. A frame with large holes may not be able to support some lenses, but a skilled optician will be able to help. Therefore, it is wise to start with the prescription before choosing a frame.
Size of the frame is as important to vision as it is to looks. Some eye doctors think that the frame should cover 20-30% of the face with the top of the frame following the line of the eyebrows. If the frame is too large, the lenses can pick up too much glare from overhead lights and distort vision. If the frame is too small, the field of vision will be too restrictive.
Materials used come in different grades or levels of quality in either metal or plastic. Generally, you get what you pay for. Thin metal frames are often the lightest and most stylish, but plastic frames are usually more durable and better able to support thick lenses.
The cheapest metal frames are made from a mix of metals that include nickel. They may have a coat of color glaze that can peel or flake off in a few months. Some of the cheaper metal frames will corrode from contact with perspiration and salty body oils, which can damage the frame and irritate or discolor the skin.
The more expensive metal frames are made of titanium and carbon-graphite and are especially durable. Adding flexon, a titanium-based alloy, gives the frame a “shape memory.” If they are bent or twisted, they will spring back to their original shape. In addition, the more expensive frames usually have several coats of color glaze.
Plastic frames also have a range of quality. Propionate plastic is used in the cheaper frames and does not come in a wide range of colors. The colors are also known to fade over time. Zyl plastic is more stylish and colorful but can become brittle. Kevlar, the same strong plastic fiber used for military helmets, is durable. Newer frames made from a resin, called Optyl, can be twisted and will snap back into shape.
Fit is also very important. If glasses fit correctly, they will not rub or irritate the skin either on the nose or behind the ears. The nose supports about 90% of the weight of the glasses, so the bridge of the frame is a big factor in determining how comfortable the glasses will feel.
The saddle bridge is a good choice for heavier glasses. This is a single piece of plastic molded to the frame that sits along the top and sides of the nose like a saddle. This type evenly spreads the weight of the lenses.
The most common bridges are those with adjustable pads on each side of the nose. These are flexible and easy to adjust, and the soft silicone material keeps the frames from sliding down the nose.
For active people, especially children, the temples should hook snugly around the ears but not be so thick that they block vision. Unlike standard hinges that open to a set distance, flexible hinges can hold glasses tightly to the head while allowing the temples to be pulled wider so that the frames slip on or off easily.
Types of lenses include the following:
- Monofocal lenses are of one focal power to correct nearsightedness, farsightedness, or astigmatism.
- Bifocal lenses, as the name suggests, combines two focal powers into one lens. The top part of the lens corrects distance vision while the lower part allows reading or seeing objects from a foot or so away.
- Trifocal lenses adds a third power for an intermediate focus between the other two. This added convenience helps focus clearly on objects two to four feet away as is needed for computer work.
- Progressive lenses have no division lines. Instead, the focal powers change smoothly as the eyes move from top to bottom. This means that a trifocal, or intermediate, correction is built into the lens but the line is not seen. One disadvantage of the progressive lens is that it may distort vision along the bottom edges, nearing the reading section. However, newer lenses have been manufactured with less distortion.
- Non-prescription reading glasses are used by many people who do not need the stronger lenses of prescription quality. These are available at pharmacies and discount stores, usually along with sunglasses. They are cheaper than prescription lenses and do have varying strengths. They also function when worn over contact lenses that correct distance vision. The weakest corrective lenses are labeled +1.00 and the strongest are +3.00.
Lens materials for both eyeglasses and contact lenses are commonly made from the following:
- Glass may be more scratch resistant than plastic, and they can be about twice as heavy. Another drawback is that they are more prone to break or chip.
- High-resin plastic costs about the same as glass lenses but are made from a resin called CR-39. They will be a bit thicker than glass but about half the weight. However, they are prone to scratching; therefore, some manufacturers routinely put a scratch-resistant coating on them.
- High-index plastic creates a lighter and about 20% thinner lens than the high-resin plastic lenses, making them ideal for moderate or strong prescriptions. However, they do cost considerably more than high-resin plastic, but they do always come with a scratch-resistant coating and UV protection.
- Polycarbonate plastic is used for the strongest lenses available, making them the preferred choice for active kids, as well as for use in safety and sports glasses. Though these lenses are not as lightweight as the high-index lenses, they are lighter than the high-resin plastic lenses. They always come with scratch-resistant coating and UV protection.
Lens coatings and treatments are added to the basic lens materials. These include the following:
- Antireflection coating (AR) helps block the light reflected off such surfaces as pavement, water, snow, and glass. Such reflection and glare can make driving difficult, especially at night. AR is a preferred protection for the stronger prescription lenses, which often increase glare. Public speakers or people frequently photographed also find it helpful. The chemical makeup of the lenses do make them harder to keep clean and frequent, hard cleaning can rub off the coating. A moistened, lint-free cloth or a cleaning solution is the best form of cleaner to use on these lenses. The scratch-resistant coating is generally applied before the antireflection coating, which leaves the AR coating vulnerable to scratches.
- Lens edges can become bulky and unsightly. If a person is very nearsighted, the edges of the concave lenses will be thick, adding unnecessary weight to the glasses, especially if frames with large lens holes are chosen. A skilled optician can grind the edges so that they will blend into the frame.
- Photochromic lenses are chemically treated so that they automatically adjust to brightness, becoming sunglass-dark in direct sunlight and clear in dimly lit rooms. They can screen out eighty-five percent of light at their darkest and ten to fifteen percent at their lightest. However, these lenses require UV light to change color, so they will not become dark while you are driving unless the sun is shining directly on your face through an open window. Therefore, a pair of sunglasses may be needed instead. Another thing, plastic lenses do not change color as quickly or become as dark as treated glass lenses, and neither type of lens changes color as quickly or become as dark in high temperatures.
- Scratch protection means that a clear, hard coating is applied to lenses to make them more resistant to scratching. It is a good idea to check that boths sides of the lens have been treated. Care must be taken during storage of glasses with this coating as extreme temperatures cause the coating to crack and peel.
- Tinted lenses remain a constant shade in all situations. Adding color to the lenses can help if one is especially sensitive to light. They are often used for cosmetic reasons as they hide wrinkles around the eyes or simply to match an outfit. Almost any color is available as a tint. Sunglasses are often gray or brown. A yellow tint can make objects appear sharper against a blue or green background. Plastic lenses are especially adaptable to tints. These lenses are dipped into heated dye to soak up the color. If you want the shade lightened, the tint can be bleached out. Glass lenses are usually tinted by applying a colored covering to the surface. This coating can get scratched off.
- UV protection means that ultraviolet rays (both UVA and UVB) will be filtered out. High-index plastic and polycarbonate plastic lenses will already have this protection, so do not allow an unscrupulous salesperson charge you extra for the protection already provided.
The following is an example of a prescription for corrective lenses and how to read it:
- OD: -2.75 (sphere) -2.25 (cylinder) 90 (axis)
- OS: -1.75 (sphere) -2.00 +1.50 add (cylinder) 90 (axis)
- OD (oculus dexter) is the right eye, identified on some prescriptions as RE
- OS (oculus sinister) is the left eye, sometimes listed as LE
- Sphere is the correction measurement for nearsightedness or farsightedness
- Cylinder is the correction measurement for astigmatism
- Axis shows where the astigmatism correction should be on the lens – the position in degrees from horizontal. It can be anywhere from 1 to 180 degrees, with 90 degrees being the vertical (up and down) line.
The term “+1.50 add” refers to an additional lens, in this case, bifocals for close work. The numbers for “Sphere” and “Cylinder” are units of lens power called diopters, which can increase or decrease in increments of a quarter (.25) diopter. The higher the number, the greater the correction.
A person with the prescription above is nearsighted, meaning the lenses are concave. This is why the diopters are preceded by a minus sign. On a prescription to correct farsightedness, the numbers would be preceded by a plus sign and the lenses would be convex.