When near vision correction is needed along with a distance correction, contact lens wearers have an number of options: they can use near vision spectacles over their contacts, use mutifocal contact lenses, or utilitize the monovision system.
Monovision works well for people who do not use both eyes simultaneously and who therefore lack stereoscopic binocular vision. In this situation it is possible to alternately "see" with one eye at a time. For those with normal stereo vision, monov ision could cause a potentially significant loss of depth perception. (See the EyeCare Report: Binocular Vision for details.) Some people can "retrain" their brains to giv e up stereoscopic vision by alternately suppressing the information from one eye at a time. The success rate in converting stongly binocular people to monovision is poor. Even if adaptation is achieved, there are some potential problems that may be encoun tered.
There is some controversy among eye doctors as to whether forcing the brain to make this adjustment is really such a good idea. The loss of stereoscopic depth perception could put the wearer at risk of misjudging distances while driving a car. Many practi tioners recommend against the use of monovision while operating a motor vehicle. Another concern is the appropriateness of altering brain function for a cosmetic benefit, especially when considering that there are alternatives that provide good visual acu ity while preserving binocular vision.
The real risks may be overstated: someone who is heavily reliant on stereoscopic binocular vision will simply give up trying to adapt to monovision and those who easily adapt most likely did not have very good stereoscopic vision at the start.
There is also a similar "modified monovision" technique which to some degree addresses the problem of loss of depth perception. Instead of a dedicated near vision contact lens in the non-dominant eye, a multifocal lens is used. This preserves ster eopsis and depth perception at distance to a significant degree in most cases. It is not always appropriate for those requiring good stereo vision at near, for example people involved in electronic assembly or manual typesetting.
Multifocal contact lenses are the best alternative for many "emerging presbyopes", those who are just beginning to need help with near vision. For current RGP lens wearers, the transition if quite easy and very often successful. Today, there is n o reason for long time contact lens wearers to have to give up their contacts because of presyopia!