Glasses/Sunglasses
Q: What should I look for when choosing a pair of sunglasses?
A: No matter what sunglass styles or options you choose, you should insist that your sunglasses:- Block out 99-100 percent of both UV-A and UV-B radiation;
- Screen out 75-90 percent of visible light (fashion tinted lenses usually do not meet this level);
- Are perfectly matched in color and are free of distortion and imperfection;
- Have lenses that are gray, green, or brown (gray is recommended).
Q: Why is it that even if you wear glasses or contacts, when you get below the water's surface (wearing goggles), you can see almost normally?
A: Because of the higher refractive index of water, light travels more slowly and is bent more in water than in air. The effect is that nearsighted persons wearing goggles underwater can see more clearly than in air.
Q: Are glass lenses with a greater "base curve" likely to give sharper vision when I look through my glasses near the edges?
A: There are a number of factors which reduce the sharpness of vision of glass or plastic spectacle lenses. There are five specific problems, known as aberrations, which can be present with any wavelength or color of light. The base curve of lenses are chosen to reduce the two most important of these aberrations--oblique astigmatism and curvature of field. Changing the base curve of the lenses, either steeper or flatter, away from the ideal curve (or "corrected curve") will increase these aberrations. In order to make lenses thinner, a flatter base curve is sometimes selected, but an aspheric surface is used to control the aberrations.
Q: How do I know if I need bifocals?
A: The most common use of bifocals is for the treatment of presbyopia in individuals aged 40 and over. Whether or not a person has needed vision correction when younger, by the early to mid-forties, the ability to accommodate or focus the eyes has diminished. Bifocals allow the wearer to see clearly both at distance and near despite the reduced focusing ability. Bifocals may also be used to help align the eyes if a person tends to over-cross his or her eyes at near. If you are over 40 or have any difficulty performing tasks at near, ask your optometrist whether bifocals or no-line progressive addition lenses could be right for you.
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