Q. It's finally happened -- I need reading glasses! Can you help me sort through the different types of corrective lenses?
A. I don't know how old you are, but if you're over 40, there's a good chance you are like me. I have both myopia (difficulty seeing distant objects clearly) and presbyopia, which makes reading difficult.
In myopia, objects in the distance do not focus sharply on the retina -- the part of the eye that senses the image and sends it into the brain. Glasses can bend the light entering your eyes from distant objects and focus the light on the retina.
Having trouble reading is caused by an entirely different problem. When we look at something close up, as we do when we're reading, little muscles tug on our lenses to change their shape. That change causes the page we're reading to focus sharply on the retina. As we get older, our lenses stiffen and lose their flexibility; they no longer can focus near objects properly. Glasses can bend light coming off the page so that it focuses better on the retina.
If you have only presbyopia, the simplest way to regain close vision is to wear reading glasses. Many drugstores and supermarkets carry them, but off-the-rack reading glasses may not be right for you. Many people need different amounts of correction in each eye and therefore require custom glasses. Also, custom glasses allow your eyes to focus properly across the full range of the lens.
If, like me, you have both myopia and presbyopia, one option is two sets of glasses: one for distance vision and one for close vision. Or you can wear bifocals, in which the upper portion of the glass corrects for distance and the lower portion for near vision.
Another option is trifocals, which correct for middle vision in addition to distance and near vision. Trifocals may be a good choice if you spend a lot of time looking at objects at a middle distance, such as a computer screen.
Progressive lenses are another option. They combine several levels of adjustment to correct both distance and close-up vision. (I've put an illustration of different corrective lenses on my website, AskDoctorK.com.)
Contact lenses, like glasses, can correct just for myopia or just for presbyopia. For many years I wore a contact lens in one eye to see things in the distance and a different lens in the other eye for reading. Not everyone's brain can tolerate this, but mine did. Bifocal, trifocal and progressive contact lenses are also available.
Another, newer, option is adjustable focusing eyeglasses. By moving a tiny slider on the bridge of the glasses, the wearer can focus at a range of distances without zones or lines. If you are doing something that doesn't require close vision -- like playing or watching a sport -- most of the surface of your glasses can be dedicated to distance vision. Then when you read a book, most of the surface of your glasses can be dedicated to near vision.
With all of the options available, you should have no problem finding the right lenses for you.
• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com.