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Measuring Actual Focal Length

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Posted by Marty on January 10, 2001 15:57:23 UTC

I will most likely feel completely stupid when I get an answer to this one. When measuring the focal length of a primary. I have read to stand the mirror up on a wall and with some light source behind you focus it on a sheet of paper. The distant betweent the mirror and paper is divided my two. Why do you divide this measurementby two? I would think that the actual measurement would be the focal length. But that is why I am sure there is a simple anwser.

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