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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.
Binoculars using either a Schmidt–Pechan roof prism, Abbe–Koenig roof prism or an Uppendahl roof prism benefit from phase coatings that compensate for a loss of resolution and contrast caused by the interference effects that occur in untreated roof prisms.
These models are often called adjustable objective (AO or A/O for short) models. Occasionally, a side-focus design (see below) might be used with a fixed reticle within the eyepiece , onto which the second focal plane (SFF) of the target image is shifted by an adjustable erector lens group .
Incomitant strabismus cannot be fully corrected by prism glasses, because the eyes would require different degrees of prismatic correction dependent on the direction of the gaze.
Prism correction. Prism lenses (here unusually thick) are used for pre-operative prism adaptation. Eye care professionals use prism correction as a component of some eyeglass prescriptions. A lens which includes some amount of prism correction will displace the viewed image horizontally, vertically, or a combination of both directions.
An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides.
A corrective lens is a transmissive optical device that is worn on the eye to improve visual perception. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye.
Adjustable-focus eyeglasses might be used to replace bifocals or trifocals, or might be used to produce cheaper single-vision glasses (since they do not have to be custom-manufactured for every person).
Joshua D. Silver is a British physicist whose discoveries have included a new way to change the curvature of lenses, with a significant application for the low-cost manufacture of corrective lenses and adjustable spectacles, especially in low-income countries.
A prism sight or prismatic sight, sometimes also called prism scope or prismatic scope, is a type of telescopic sight which uses a reflective prism for its image-erecting system, instead of the series of relay lenses found in traditional telescopic sights.