Overconvergence , when the eyes work too hard to see close up, is one. Regardless of the reason you need a prescription for near-vision correction, bifocals all work in the same way. A small portion of the eyeglass lens is reserved for the near-vision correction. The rest of the lens is usually a distance correction, but sometimes has no correction at all in it. Generally, you look up and through the distance portion of the lens when focusing on points farther away, and you look down and through the bifocal segment of the lens when focusing on reading material or detail work up to about 18 inches away.
Computers are an excellent example of something that is in a person's intermediate zone.
Motorists who need to see in the distance to drive, to see the gauges on the dashboard, and to read a map also would benefit from a trifocal. Flat-top and Executive lens styles are the most common trifocals. How Multifocals Are Fitted Bifocals are typically placed so the line rests at the same height as the wearer's lower eyelid. As a bifocal wearer drops his eyes downward to read, the eyes naturally seek out the near-vision portion of the lens. Trifocals are fitted a bit higher, with the top line of the intermediate area placed even with the pupil .