Adjustable focus eyeglasses are prescription eyeglasses with an adjustable focal length. They compensate for refractive errors (such as presbyopia) by providing variable focusing, 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. Adjustable focus eyeglasses have one focal length, but it is variable without having to change where one is looking.
Possible uses for such glasses are to provide cheap glasses for 3rd world countries, or to accommodate for presbyopia.
There are currently two basic methods to achieve variable focal length: electro-optical and opto-mechanical.
Early work on opto-mechanical methods was done by Martin Wright. Opto-mechanical spectacles allow focus 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 focus at different distances (from infinity up to 13"). The appropriate addition range depends on the user’s level of refractive error.[clarification needed] 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 focus of a lens. Stephen Kurtin also has a product based on what appears to be a related design called TruFocals.
Unlike with bifocals, near-vision correction is achieved over the entire field of view, in any direction. Distance vision corrections are made by re-adjusting the lens for distance, instead of by tilting and/or rotating the head to view object through the best part of the lens for the distance. Adjustable focus lenses, like single-focus lenses, also reduce image-jump and spatial distortion in the field of view associated with traditional multi-focal lenses. Additionally, the ideal near-vision correction can be achieved with precision, because the variable lenses emulate the focusing action of the youthful (non-presbyopic) eye.
The adjustable focus mechanism is controlled by a mechanism located on the glasses, requiring adjustment every moment the user wants to change from seeing near to far (or vice versa).
See also 
- Adaptive optics have been suggested as another potential for adjustable eyeglasses.
- Intraocular lens "CrystaLens" replaces the normal eye lens with an adjustable one that is adjusted by using the eye's focusing muscles to focus.
- Electrowetting is a technology used to electrically adjust the path of light
- http://www.gizmag.com/go/5516/ University of AZ researchers work on a prototype that switches lens strength on and off
- US Patent 3,598,479 and Trans Ophthal Soc UK (1978) 98, 84
- *Focusspec – appears to be an example of this
- US patent is Berreman 4,190,330
- http://science.slashdot.org/story/09/08/04/0148219/Adjustable-Focus-Glasses-Can-Replace-Bifocals?from=rss see comments
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