Adjustable focus

Adjustable focus eyeglasses compensate for the effects of presbyopia, lack of focusing, by providing on-demand clarity at any desired distance.

Two approaches can be found in the literature: electro-optical (often utilizing liquid crystals as the active medium) and opto-mechanical. As to the former, [US patent is Berreman 4,190,330] ; to date no commercial electro-optical eyeglasses have been announced. As to the latter, perhaps the most comprehensive early work was that of Wright [US Patent 3,598,479 and Trans Ophthal Soc UK (1978) 98, 84] unfortunately fell short of practicality.Opto mechanical,adjustable focus spectacles, offer on-demand adjustable sometimes called variable focus over the entire lens area. Focus is controlled 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.

Adjustable focus eyeglasses are suitable for a wide range of every day needs and environments. They combine high quality rigid ophthalmic lenses carrying the user’s complete distance eyeglass prescription, with flexible lenses that change shape to deliver on-demand optical add power thereby enabling sharp focus at every distance from infinity up to 13". The appropriate addition range depends on the user’s level of presbyopia, which is closely related to age, and can be as high as 3.00 diopters. A tiny mechanism, actuated by the slider, simultaneously controls both flexible lenses to assure appropriate ADD tracking in both eyes.

Bifocals and progressive lenses are static in that the user has to change their eye position to look through the appropriate zone power corresponding to the desired focal point. Typically up, through the top of the lens for distance and down, through the bottom of the lens for near.

History

The youthful human eye provides dynamic focus. Presbyopia is the age-related loss of accommodation. The notion that adjustable focus glasses would provide ideal vision for presbyopes has long been recognized. Achieving that goal, however, has proven difficult primarily because of optical and mechanical restraints.

Advantages

Optimal vision over the entire field of view, in any direction, and at every distance, far, near orin-between.

The ideal ADD power can be achieved with precision because the dynamic lenses simulate the focusing action of the youthful human eye.Adjustable focus spectacles also eliminate;
* image-jump and spatial distortion in the field of view, created by static multifocal lenses; people are less likely to trip when walking up or down steps.
* the distortions inherent in progressive variable addition lenses (PALs).
* head tilt and rotation associated with bifocals and progressive addition lenses, in order to view object clearly.

Disadvantages

The adjustable focus mechanism is controlled by a slider located on the bridge which spans the user’s nose; these glasses are therefore not suitable for use when wearing a helmet for example, when riding a motorcycle.

References

* Progressive Addition Lenses: History, Design, Wearer Satisfaction and Trends Pope, D R OSA TOPS Vol. 35, Vision Science and Its Applications, 2000
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* The correction and management of ametropia in older patients David B Elliott PhD, MCOptom, FAAO Investigative Ophthalmology and Visual Science. 2004;45:2122-2128.)
* Stepping Up to a New Level: Effects of Blurring Vision in the Elderly Heasley K, Buckley J G,Elliott D B
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