Computer glasses are prescription glasses that are designed to wear when doing computer work. They allow you to focus your eyes on a computer screen, which is farther away than reading material is normally
held. There are other activities such as for example playing piano(piano glasses) or shooting (shooting glasses) that also require a similar custom fitting. These specialty glasses called "task-specific"
are designed to meet the visual needs of the activity. General purpose bifocals are not designed for computer work. They force the wearer to tilt the head back in order to focus on the screen while looking through the lower segment of the bifocal lenses. Such a forced
position can cause neck and shoulder pain and it can also cause arm and
low back pain. Depending on the individual's vision and type of
work, there are several options while selecting computer-specific
glasses, such as Monofocal ,Bifocal ,Trifocal and Progressive Addition Lenses (PAL) Source: http://www.ccohs.ca/oshanswers/ergonomics/office/computer_glasses.html