Variable focus optics are required in a number of applications. For most of these situations mechanical movement of the lens is required to change the focal point. In spectacles this is not practical and multi-focus lenses are used. A good example of multi-focal lenses is the bifocal lens which corrects for presbyopia with a + diopter lens. Since the accommodation range is reduced with age, this requires frequent change in the refractive prescription. In addition, the transition between the - diopter correction for myopia and the + diopter correction for presbyopia leaves certain focal ranges out of focus. Multi-focal lenses, such as trifocal lenses, attempt to correct for this limitation with limited success.
In many other optical applications the resolution is limited by how rapidly the focal length can be corrected. The input optical waveform may be distorted by the medium refractive variations. In this situation the focal length and/or refractive correction should be changed dynamically as a function of the time varying distortion. Electro mechanical methods have been used for this application, however, they are primarily applied to reflective optical systems.