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Actually guys it doesn't quite work that way. If you go up one f/stop on a camera lense (same thing applies to telescopes), say f/4 - f/5.6 then you double the exposure time. The next step up is f/5.6 to f/8, which is double again. So if you go from f/4 to f/8 you will need 4 times the exposure time f/8 to achieve the same exposure at f/4.
Sorry to be the bearer of bad tidings.
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