AVS Addicted Member
Join Date: Dec 2000
Location: brookeville, maryland, usa
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Every lens has an effextive f stop. Its the way lenses are speced. a fixed lens has a maximum effective f stop. . a zoom lens can have a constant F stop at both ends of its zoom, but to do so the lens has to be much wider than the zoom lenses used on projectors So as just a relative example, at close zoom, say the F stop is 2.8 (just making a number up) and a long zoom its is 4.0. Those numbers mean the lens will transmit 50% less light at long zoom than at short zoom. The bigger nger the zoom ratio, the long throw multiplier divided into the short throw multiplier, in the case of the JVCs, 2.8/1.4 equals 2, the more light one will lose from going to from short to long. Think of it has at short throw, the image coming out of the lens is as big as it can get. As you zoom towards long, increasing the throw dfistance, the size of the image must get smaller in order not to overfill the screen area.. this means less glass in the lens is being used. sort of like sending water doiwn a 12 inch pipe and then switching to a 6 inch pipe (diameter). Holding the pressure constant, the pipe can only transmit 1/4 as much water. pi r squared to determine the area. Effective F stops are a log funnction and the sequence for each 50% cut is 2, 2.8, 4, 5.6, 8, 11, 14. some of the calculators use the effective f stops at each end and then use a log calculators, others are much more crude.
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