LYCOS RETRIEVER 
Lens: Focal Length
built 621 days ago
A lens system consisting of two systems combined so that the back focal point of the first (the objective) coincides with the front focal point of the second (the ocular) is called a telescope. Parallel entering rays leave the system as parallel rays. The magnification is equal to the ratio of the focal length of the first system to that of the second.
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The new lens works with both Nikon's film and digital SLRs, and offers a 157.5mm equivalent focal length when used on a Nikon DX format sensor. Construction is 14 elements in 12 groups (of which there are one ED glass and one Nano Crystal Coat-deposited lens element), a nine-blade rounded diaphragm, and internal focusing, plus a bayonet lens hood (included in the bundle along with a soft lens pouch). The lens offers a reproduction ratio of 1:1, and focusing to 31cm (12.2 in).
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Why carry around a whole bag of fixed focal length ("prime") lenses when you could just buy a Tamron 28-300 zoom lens for less than $400? With a twist of a ring, the Tamron will give you any focal length from 28mm to 300mm. The only problem with this idea is that, sadly, the laws of physics and common sense have not been repealed.
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This requires a long focal length positive lens for adequate magnification, say 1/2 meter, 1 meter, or many meters. This arrangement is often used for astronomical photography. It may seem paradoxical to one unfamiliar with optics that in this application, a weaker, lower power lens (long focal length) gives the greatest magnification.
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PENTAX ... expands its limited lens series with the smc PENTAX DA 35mm f/2.8 Macro Limited (focal length equivalent to 52.5mm in the 35mm format). This compact lens with one-to-one magnification in a normal focal length features high-grade aluminum construction of the lens barrel, hood, and cap.
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If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is ... called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens.
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Focal Length