When do you need glasses?

If you need glasses, take heart. "Eyewear" has come a long way since the days of ugly utilitarian frames and bottle-glass lenses. "Glasses" are so-called because originally the lenses were made of glass; nowadays they are usually made of a special lightweight plastic.

New materials with extra light-bending capacity make today's high-powered lenses thinner, lighter and less distorting when viewed from the front. Aspheric lenses, in which the curvature changes gradually, bulge less and provide better vision with less distortion at the edges.

They are suitable for correcting both near and farsightedness. If your prescription includes a correction for astigmatism, the lenses will be specially ground so that they bend light more in one direction than another, to neutralize the irregular curvature of your cornea. These are called toric lenses. You can have your lenses coated with a variety of finishes:

• Anti-reflection coatings can be useful for computer work or night driving; they can also make thicker lenses look thinner.
• Scratch-resistant coatings protect against damage and are especially useful for children and sports players.
• A scratch-resistant coating can be combined with a coating specially designed to protect your eyes against ultraviolet (UV) rays in sunlight — though some lens materials have inherent UV protection.

Before you consider your choice of frame and lens coating, you should know what type of glasses you require.

• Single-vision lenses are the simplest type of glasses in that they correct for nearsightedness or farsightedness with a single prescription.
• Manufactured lenses in glasses (and contact lenses) work in exactly the same way as the lens in your eye: they bend light rays.
• If you are farsighted, a convex lens — which is thicker in the centre than at the edges just like the lens in your eye — adds to the focusing power of your own eye to pull the focal point forward onto the retina so that near images appear sharp again.
• If you're nearsighted, your lens will be concave — thinner in the centre than at the edge.
• This bends light rays outwards, compensating for the excess bending produced by your eyes and pushing the focal point back toward the retina, so that distant objects come back into focus.

• If you have presbyopia, a pair of reading glasses may be the answer. Even if you also require a prescription for distant vision, you could have a separate pair of reading glasses for close work.
• If you usually wear contact lenses for distant vision, your optometrist can prescribe reading glasses that will work while you are wearing your lenses.
• You can buy simple magnifying reading glasses over the counter without a prescription. Since presbyopia tends to worsen with time you may need to wear increasingly strong reading glasses as you get older.

• They enable two different prescriptions to be combined in a single lens, for people who need one correction for regular wear and another for close work.
• The upper part of the lens corrects distant vision and the lower part close vision, with a distinct dividing line between the two halves.
• There are also trifocals, which have three sections for distant, intermediate and near vision. However, bifocals and trifocals do not suit everyone and the sudden jump from one correction to another can be irksome, so varifocals are now more often prescribed.

They operate similarly to bifocals for people who need more than one correction but, as the name implies, use a graduated correction in which the power of the lens changes smoothly from one prescription to the other without visible divisions.