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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. Contact lenses are worn directly on the surface of the eye.
The f-number N is given by: = where f is the focal length, and D is the diameter of the entrance pupil (effective aperture).It is customary to write f-numbers preceded by "f /", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N. [1]
The first contact lenses were made of glass, in 1888. Initially the glass was blown but soon lenses were made by being ground to shape. For the first fifty years, glass was the only material used. The lenses were thin, yet reports of injury were rare. In 1938 perspex (polymethylmethacrylate, or PMMA) began to replace glass in contact lens ...
In microscopy, NA is important because it indicates the resolving power of a lens. The size of the finest detail that can be resolved (the resolution) is proportional to λ / 2NA , where λ is the wavelength of the light. A lens with a larger numerical aperture will be able to visualize finer details than a lens with a smaller numerical ...
Canon EF 70–200mm f/2.8L IS II USM with lens hood. The EF 70–200mm lenses are a group of telephoto zoom full-frame lenses made by Canon. The lenses have an EF mount to work with the EOS line of cameras. The lens comes in seven different versions, all of which have fixed maximum aperture at all focal lengths, and are L-series lenses. f/4L ...
Leica 50 mm f /2.0 Summicron-R 2nd version – 1977 (built-in lens hood, 3-cam and R-cam only version) Leica 50 mm f /1.4 Summilux-R 1st version; Leica 50 mm f /1.4 Summilux-R 2nd version; Leica 50 mm f /1.4 Summilux-R 3rd version – 1997 (ROM contacts) Leica 60 mm Macro-Elmarit-R 1st version – 1972 – outside bayonet lens hood fitting
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