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Message: <blockquote data-quote="Dave_W" data-source="post: 98046" data-attributes="member: 9521">I want to use my variable ND filter to take a photograph of a busy street corner and want the exposure to be at least 15 to 20 min long so that it appears to be a completely vacant of people. But the problem I have is back calculating the length of exposure to ND without having to do several 20 min tests. I've found sites what give simple back calculations but nothing that would cover a lengthy exposure. I wonder if I could do some tests and then on a piece of graph paper extrapolate back what exposure to ND would get me to 20 mins? Are there any former mathematicians in the audience? Or better yet, computer programmers that can write a simple program for such calculations?</blockquote>