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Refractive surgery is an optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses.This can include various methods of surgical remodeling of the cornea (keratomileusis), lens implantation or lens replacement.
This means the lens's F-number value is fixed to the overall designed focal ratio of the optical system (the diameter of the primary mirror divided into the focal length). The inability to stop down the lens results in the catadioptric lens having a short depth of field.
HCD lenses are optimized for slightly smaller image sensors covering 48×36 mm (1.9×1.4 in), [95] so the crop factor for HCD lenses is slightly greater at 0.72. With these sensors, an 80 mm lens would have the equivalent coverage of a lens with a focal length of approximately 60 mm on a "full-frame" camera.
The shallow depth of field provided by a telephoto lens also allows the subject of the photography to stand out better against the background. [11] Infrared corrected special-low-dispersion glass also has benefits to CCTV cameras. The low chromatic aberration of SLD glass allows the lens to always stay in focus, from visible light to infrared. [12]
French Varilux Logo. Varilux is a brand name belonging to Essilor International, a producer of corrective lenses.The first version of the lens was invented by Bernard Maitenaz and released in 1959, and was the first modern [clarification needed] progressive lens to correct presbyopia.
Polymethyl methacrylate (PMMA), [6] commonly known as acrylic, is a transparent thermoplastic available for use as ocular prosthesis, replacement intraocular lenses when the original lens has been removed in the treatment of cataracts and has historically been used as hard contact lenses.
For the simplified scenario of visible light travelling from air (n 0 ≈ 1.0) into common glass (n S ≈ 1.5), the value of R is 0.04, or 4%, on a single reflection. So at most 96% of the light (T = 1 − R = 0.96) actually enters the glass, and the rest is reflected from the surface. The amount of light reflected is known as the reflection loss.
If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela.