These reading glasses typically come in a power of plus 1 diopter, which is the lowest strength, up to plus 3.5 diopters. The reading glasses' power increases in increments of .25 diopters. Drugstore reading glasses are cheap and convenient, allowing presbyopics the freedom to easily purchase a new pair if they lose the old one, or to have several around the house and office. One rule of thumb when selecting a pair on your own is that they're too weak if you have to hold the reading material far away, and too strong if you have to hold it uncomfortably close to your face.