In general, if your glasses are too weak or too powerful, and you're not keeping your reading material in focus, you may have eyestrain and maybe a headache. You might have the same strength but hold the reading materials at the wrong distance and have the same outcome. There are two types of lenses in glasses: hard and soft. Hard lenses are very thin and are used for safety glasses, while soft lenses are thicker and are used for reading glasses. People usually need stronger reading glasses than regular sunglasses because they can't see as well without their glasses on. However, some people with strong eyesight can wear weaker reading glasses because they don't have to worry about looking out into the sunlight. The strength of lenses is measured in diopters; the higher the number, the stronger the lens. A person who needs +1.00-diopter lenses would fit into a pair of +3.00-diopter glasses. The type of lens used in glasses affects how they fit on the eye. With hard lenses, there's less room for error because you can't put them on too tight or they'll hurt when you remove them. With soft lenses, you can get away with putting them on too tight since they'll still mold to the shape of the eye when you take them off.