There are currently two basic methods to achieve variable focal length: electro-optical and opto-mechanical. Electro-optical often uses liquid crystals as the active medium. Applying an electric potential to the liquid changes the refraction of the liquid. Early work on opto-mechanical methods was done by Martin Wright. Opto-mechanical spectacles allow<unk>  control by the wearer via movement of a small slider located on top of the bridge. The user adjusts the lens for optical clarity at the desired distance. They are a combination of rigid and flexible lenses that can change prescription to enable sharp<unk>  at different distances (from infinity up to 13"). The appropriate addition range depends on the user’s level of refractive error. A tiny mechanism, actuated by the slider, simultaneously controls both flexible lenses to assure appropriate near vision tracking in both eyes. Another type of opto-mechanical lens is the design of Joshua Silver, and uses liquid pressure against a diaphragm to control<unk>  of a lens. These lenses were meant to provide improved vision without prescription by an optometrist, since these professionals are in short supply in many countries. Each eyepiece encloses a reservoir of fluid silicone and the user adjusts the level of fluid with a dial until they are satisfied with the result. Stephen Kurtin also had a product based on what appears to be a related design called Superfocus (originally TruFocals). This company has since gone bankrupt.
Adjustable-focus eyeglasses
Browline glasses
Windsor glasses
Trifocal lenses
Argus retinal prosthesis
Multifocal intraocular lens
Ocular prosthesis
Pinhole glasses