Adjustable-focus eyeglasses

Methods

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 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 various 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 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.

Stephen Kurtin also had a product based on what appears to be a related design called Superfocus (originally TruFocals). The company has since gone bankrupt.

Advantages

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.

Disadvantages

The focal distance is changed by a mechanism located on the glasses, requiring periodic adjustment as the user switches their gaze to nearer or farther objects.

References

References

1. Hanlon, Mike. (April 15, 2006). "Eyeglasses with adaptive focus". New Atlas.
2. "Variable focus liquid lenses".
3. Wright, Basil M.. (1978). "Variable focus spectacles". Transactions of the Ophthalmological Societies of the United Kingdom.
4. *[http://www.focusspecs.com Focusspecs] – appears to be an example of this {{Webarchive. link. (2020-02-22)
5. US patent is Berreman 4,190,330
6. Bakalar, Nicholas. (September 26, 2011). "Self-Adjustable Eyeglass Lenses". [[The New York Times]].
7. Markoff, John. (2009-08-03). "Making Eyeglasses That Let Wearers Change Focus on the Fly". New York Times.
8. (2009-08-04). "Adjustable-Focus Glasses Can Replace Bifocals". Slashdot.