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For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens.For a converging lens (for example a convex lens), the focal length is positive and is the distance at which a beam of collimated light will be focused to a single spot.
Color positive picture (A) and negative (B), monochrome positive picture (C) and negative (D) In photography, a negative is an image, usually on a strip or sheet of transparent plastic film, in which the lightest areas of the photographed subject appear darkest and the darkest areas appear lightest. [1]
Using a positive lens of focal length f, a virtual image results when S 1 < f, the lens thus being used as a magnifying glass (rather than if S 1 ≫ f as for a camera). Using a negative lens (f < 0) with a real object (S 1 > 0) can only produce a virtual image (S 2 < 0), according to the above formula.
However, the traditional sign convention used in photography is "real is positive, virtual is negative". [1] Therefore, in photography: Object height and distance are always real and positive. When the focal length is positive the image's height, distance and magnification are real and positive. Only if the focal length is negative, the image's ...
The telephoto lens configuration combines positive and negative lens groups with the negative at the rear, serving to magnify the image, which reduces the back focal distance of the lens (the distance between the back of the lens and the image plane) to a figure shorter than the focal length. This is for practical, not optical reasons, because ...
Other lenses for the Contax included the Biotar, Biogon, Orthometar, and various Tessars and Triotars. The last important Zeiss innovation before the Second World War was the technique of applying anti-reflective coating to lens surfaces invented by Olexander Smakula in 1935. [8] A lens so treated was marked with a red "T", short for "Transparent".
Converging lenses have positive optical power, while diverging lenses have negative power. When a lens is immersed in a refractive medium, its optical power and focal length change. For two or more thin lenses close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: P = P 1 ...
The earliest double Gauss lens, patented by Alvan Graham Clark in 1888, consists of two symmetrically-arranged Gauss lenses. Each Gauss lens is a two-element achromatic lens with a positive meniscus lens on the object side and a negative meniscus lens on the image side. In Clark's symmetric arrangement, this makes four elements in four groups ...