<unk>  are<unk>  with an<unk>  focal length. They compensate for refractive errors (such as presbyopia) by providing variable<unk> ing, allowing users to adjust them for desired distance or prescription, or both. Current bifocals and progressive lenses are static, in that the user has to change their eye position to look through the portion of the lens with the focal power corresponding to the distance of the object. This usually means looking through the top of the lens for distant objects and down through the bottom of the lens for near objects.<unk>  have one focal length, but it is variable without having to change where one is looking. Possible uses for such glasses are to provide inexpensive<unk>  for people from low-income groups, developing countries, third world countries or to accommodate for presbyopia.
Adjustable-focus eyeglasses
Browline glasses
Windsor glasses
Trifocal lenses
Argus retinal prosthesis
Multifocal intraocular lens
Ocular prosthesis
Pinhole glasses