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Forum: General Photography
06-18-2015, 11:36 AM
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Many elements are required for wide lenses and long lenses although there are exceptions. Wide lenses have problems with the oblique aberrations and long lenses with color correction. It is difficult to accomplish this with only 4 elements unless the designer has a huge budget for expensive glass and uses aspheric surfaces.
One of the biggest misconceptions in photography is curvature of field. It is an aberration at the focal plane, where the image is not flat but curved. All lenses must be corrected for it so the image lays flat on the film/sensor. Having a flat subject plane is another issue. Some macro lenses do this and also projection lenses.
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Forum: General Photography
06-17-2015, 02:44 PM
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The classic combination with fluorite is to use a high index Barium glass. The other element could possibly be Lanthanum. Some of these combinations yield near zero tertiary spectrum (residual color). A major problem with long photographic lenses is that color passing through the marginal area of the lens does not focus at the same plane as that same color passing through the paraxial area of the lens. This leads to longitudinal chromatic wide open but much reduced when stopped down. If you have a diaphragm, that can be used to truncate the poorly corrected marginal rays and reduce fringing. If however you have no diaphragm (telescope) then this higher order aberration (spherochromatism) must be solved for wide open shooting.
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Forum: General Photography
06-17-2015, 11:05 AM
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An f/7.1 lens/scope can perform very well as we've see but so can an f/1.4 lens if the designer has the time and money to do so. I would bet that the scope design uses expensive, exotic glass (maybe Barium combined with a super low dispersion glass) to help solve the longitudinal and lateral color. The 3 elements are probably air spaced to give the designer more degrees of freedom in correcting the 5 remaining aberrations. Physics tells us that the diameter of the optic does have an affect on performance. A 10 inch diameter lens will outperform a 2 inch diameter one if aberrations are corrected to the same degree in each. This is the main reason why telescopes are made with large diameters. The other reason being light gathering ability.
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