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Roof prism designs allow a simpler exterior, and are now common but they are more expensive to produce. Complicating production requirements make high-quality roof prism design binoculars relatively costly to produce compared to in optical quality equivalent Porro prism binoculars.
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.
Roof prism designs result in objective lenses that are almost or totally in line with the eyepieces, creating an instrument that is narrower and more compact than Porro prisms and lighter. There is also a difference in image brightness.
There are three primary types of optical telescope: Refracting telescopes, which use lenses and less commonly also prisms ( dioptrics) Reflecting telescopes, which use mirrors ( catoptrics) Catadioptric telescopes, which combine lenses and mirrors.
Although the Spotmatic's stop-down (manual diaphragm lenses) system was less convenient than the RE Super's open aperture (auto-diaphragm lenses) system, the Spotmatic's two CdS cells on either side of the eyepiece reading off the focusing screen was less expensive and complex than the RE Super's system (see above), and thereby more popular.
Since plastic Fresnel lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used to concentrate sunlight for heating in solar cookers, in solar forges, and in solar collectors used to heat water for domestic use.