When macular degeneration impairs macroscopic vision,
*magniscopic* vision may help. The ability of lenses to magnify is a function of their focal length, which is measured in diopters:
diopter
n.
5Fr dioptre < L dioptra < Gr instrument for leveling, etc. < dia3, through + base of opsis, sight < ops, EYE6 a unit of measure of the power of a lens, equal to the power of a lens with a focal length of one meter
di[op4tral
adj.

If I remember correctly, a four diopter lens has a focal
length of a quarter of a meter.(Hey, Bean, am I right?_)
So I have a six inch four diopter lens to help me read my
monitor.