Reading area refers to the part of a spectacle lens that is used for close work. With single vision reading glasses this is the whole lens, whereas with bifocals the reading area forms some or all of the lower half of the lens depending on the type chosen. For example a round 24 bifocal means the reading area is round in shape and 24mm in diameter, whereas a D35 is flat topped and 35mm wide.

With varifocals the reading area varies enormously both in how far below the pupil centre it begins and also in its width. In general terms the better quality the varifocals the wider the reading area and the easier the lenses are to get used to – essentially you get what you pay for. In order to fit the whole reading area into a small shallow frame it is necessary to choose a varifocals with a short corridor length.