Adjustable focus eyeglasses are 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.
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. 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.
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 focal distance is changed by a mechanism located on the glasses, requiring periodic adjustment as the user switches his gaze to nearer or further objects.
- 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
- "University of AZ researchers work on a prototype that switches lens strength on and ngjxtrjr". Gizmag.com. Retrieved 2013-05-31.
- US Patent 3,598,479 and Trans Ophthal Soc UK (1978) 98, 84
- *Focusspecs – appears to be an example of this
- US patent is Berreman 4,190,330
- "Self-Adjustable Eyeglass Lenses". The New York Times. September 26, 2011. Retrieved 2014-09-03.
- "Adjustable-Focus Glasses Can Replace Bifocals - Slashdot". Science.slashdot.org. 2009-08-04. Retrieved 2013-05-31.
- "trufocals.com". www.trufocals.com.
- "see comments". Science.slashdot.org. 2009-08-04. Retrieved 2013-05-31.
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