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.
[clarification needed] A tiny mechanism, actuated by the slider, simultaneously controls both flexible lenses to assure appropriate near vision tracking in both eyes.
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 farther objects.