Telephoto and retrofocus lenses

Very long telephoto lenses often work like a telescope with a Barlow lens (Figure 7.12, upper right). Technically speaking, telephoto means a lens whose focal length is much longer than its physical length, and the classic achromat-with-Barlow design is the standard way of achieving this, although asymmetrical triplets and asymmetrical double Gauss designs can do the same thing to a lesser degree.

The opposite of a telephoto is a retrofocus wide-angle lens, one whose lens-to-film distance is longer than its focal length. To leave room for the mirror, the lens of an SLR can be no closer than about 50 mm from the sensor. To get an effective focal length of, say, 28 mm, the wide-angle lens has to work like a backward telescope; it is a conventional lens with a large concave element in front of it.

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