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Most of the gravitational lenses in the past have been discovered accidentally. A search for gravitational lenses in the northern hemisphere (Cosmic Lens All Sky Survey, CLASS), done in radio frequencies using the Very Large Array (VLA) in New Mexico, led to the discovery of 22 new lensing systems, a major milestone.
Simulated animation of globe effect (right) compared with a simple pan (left) In photography, distortion is particularly associated with zoom lenses, particularly large-range zooms, but may also be found in prime lenses, and depends on focal distance – for example, the Canon EF 50mm f / 1.4 exhibits barrel distortion at extremely short focal distances.
Uses an equiconvex crown glass lens (i.e. R 1 > 0 with −R 1 = R 2) and a complementary-curved second flint glass lens (with R 3 = R 2). The back of the flint glass lens is flat ( R 4 = ∞). A Littrow doublet can produce a ghost image between R 2 and R 3 because the lens surfaces of the two lenses have the same radii.
In a circular fisheye lens, the image circle is inscribed in the film or sensor area; in a diagonal ("full-frame") fisheye lens, the image circle is circumscribed around the film or sensor area. This implies that using a fisheye lens for a different format than it was intended for is easy (as opposed to a rectilinear lens), and may change its ...
In order to see a clear image, the eye must focus rays of light on to the light-sensing part of the eye - the retina, which is located in the back of the eye.This focusing - called refraction - is performed mainly by the cornea and the lens, which are located at the front of the eye, the anterior segment.
Like other lenses for vision correction, aspheric lenses can be categorized as convex or concave. Convex aspheric curvatures are used in many presbyopic vari-focal lenses to increase the optical power over part of the lens, aiding in near-pointed tasks such as reading. The reading portion is an aspheric "progressive add".
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