Photochromic. Photochromic lenses are chemically treated so that they automatically adjust to brightness. They get sunglass-dark in direct sunlight and clear in a dimly lit room. They can screen out up to 85 percent of light at their darkest and 10 percent to 15 percent at their lightest.
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| Glasses That Improve Vision |
Some cautions about photochromic lenses: These lenses require UV light to change color, so they won't get dark while you're driving unless the sun is shining on your face through an open window. You may need to keep a pair of sunglasses in the car. Plastic don't change color as quickly or get as dark as treated glass lenses. And neither type of lens changes color as quickly or gets as dark in high temperatures.
Tint. Unlike photochromic lenses that respond to various levels of brightness, tinted lenses remain a constant shade in all situations. Adding color to your glasses can help if you're especially sensitive to light and wish to use your lenses as sunglasses. You may also want to hide wrinkles around your eyes or simply make a fashion statement. Almost any color can be chosen for 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. The 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, however, are usually tinted by applying a colored covering to the surface. This coating can get scratched off.
Lens edge. If you're very nearsighted, the edges of your concave lens will be thick. This extra bulk can look unattractive and add unnecessary weight to your glasses - especially if you choose frames with large lens holes. A skilled optician can grind the edges so that they blend into the frame.
The frames
When you're looking for new glasses, you may be tempted to start with frames on the display rack. If you want to save time, start with your prescription. Some types of lenses won't work with certain frames. For example, if your prescription calls for thick lenses, a thin wire frame might not be able to support them. A frame with large lens holes may make your glasses too heavy. A skilled optician can tell by your prescription what kinds of frames will work for you, narrowing your search.
Size. The size of your frame can be important for your vision as well as your looks. Some eye doctors think that the frame should cover 20 percent to 30 percent of your face, with the top of the frame following the line of your eyebrows. If your frame is too large, the lenses can pick up too much glare from overhead lights and distort your vision. If the frame is too small, your field of vision may be more limited than you'd like.
If you need strong - and therefore thick - lenses, try smaller frames. They'll reduce the weight of your glasses and may eliminate distortion created when your lenses extend beyond your field of vision.
Materials. Frames come in different grades - or levels of quality - of metal and plastic. Generally you'll get what you pay for. If you buy the least expensive metal or plastic frame, you'll likely get a lower-quality material. Thin metal frames are usually 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 get 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. This corrosion can damage the frame and irritate or discolor your skin.
The more expensive metal frames made of titanium and carbon-graphite are especially durable. And Flexon, a titanium-based alloy, has "shape memory." You can bend and twist it, and it springs back into shape. The more expensive frames usually get several coats of color glaze. If you live in a warm climate or have a job where you perspire a lot, you might opt for the high-quality metal frames or plastic frames that won't corrode.
Plastic frames, like metal frames, have a range of quality. Propionate plastic is used in the cheaper frames. It doesn't come in a wide range of colors, and these colors are 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. And the newer frames made of a resin called Optyl can be twisted around your finger and snap back into shape.
Fit. If your glasses fit correctly, they'll feel snug and secure, yet they won't rub behind your ears or irritate the bridge of your nose. If the flames do bother you, they can be adjusted at the hinges, bridge or temples - the side arms that rest on your ears. You can also change the tilt of your glasses or adjust them closer to your face.
Your nose supports about 90 percent of the weight of your glasses. So the bridge of your frame is a big factor in determining how comfortable your glasses feel. The saddle bridge is a good choice for heavier glasses. It's a single piece of plastic molded to the frame that sits along the top and sides of your nose like a saddle, evenly spreading the weight of the lenses. The most common bridges are those with adjustable pads, with a pad sitting on each side of your nose. They're flexible and easy to adjust, and the soft silicone material keeps the frames from sliding down your nose.
For active people, such as kids, the temples should hook snugly around the ears and not be so thick that they block vision. Unlike standard hinges that open to a set distance, flexible hinges can hold your glasses tightly to your head but allow the temples to be pulled wider so that the frames slip on or off easily.
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After you get new glasses, your eyes and face may need a short adjustment period. This may take a few days or even a week. During this time you may experience some eye ache, but it shouldn't be unbearable or persistent. If it's so painful that you can't wear the glasses, or if the pain lasts more than a week, check with the optician. An adjustment to the frames will probably help. If the pain still persists, ask your eye doctor to check the glasses to make sure the prescription is right.
Regardless, it's a good idea to have the fit of your glasses checked every year or so. No matter how well built your glasses are or how careful you are with them, they easily get out of alignment. To find out more, you can check out Glasses That Improve Vision.