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| Telephoto ratio | The ratio between the overall length of a telephoto lens and its focal length is called the telephoto ratio. Put another way, it is the value of the distance from the apex of the frontmost lens element to the focal plane divided by the focal length. For telephoto lenses, this value is less than one. For reference, the telephoto ratio of the EF 300mm f/2.8L USM is 0.91 , and that of the EF 600mm f/4L USM is 0.78. |
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| Telephoto type (teletype) lens | With general photographic lenses, the overall length of a lens (the distance from the apex of the frontmost lens element to the focal plane) is longer than its focal length. This is not usually the case with lenses of particularly long focal length, however, since using a normal lens construction would result in a very large, unwieldy lens. To keep the size of such a lens manageable while still providing a long focal length, a concave (negative) lens assembly is placed behind the main convex (positive) lens assembly, resulting in a lens which is shorter than its focal length. Lenses of this type are called telephoto lenses. In a telephoto lens, the second principal point is located in front of the frontmost lens element. |
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| The high cost of synthetic fluorite crys | To achieve a high level of sharpness both at the center and out to the edges of an image when shooting with a telephoto lens, it is desirable for the index of refraction of the front convex lens element to be as small as possible. Accordingly, the use of fluorite with its low index of refraction effectively improves image quality over the total image area. |
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| Thorough elimination of the secondary sp | When a convex fluorite lens is combined with a concave wide-dispersion optical glass lens to correct red and blue wavelengths, the partial dispersion characteristics of the fluorite also effectively compensate the green wavelength as well. This greatly reduces the presence of secondary spectrum and brings all three wavelengths ~ red, green and blue ~ together at the same focal point to realize virtually ideal chromatic aberration compensation, (apochromatic performance). |
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