In a message dated 9/17/98 3:09:59 AM Eastern Daylight Time, gma@xxxxxxxxxx
writes:
<< Rule of thumb: Length of tube for 1:1 = half the focal length of the
lens, if the lens goes to 1:2 by itself. Eg for the 50 macros, 25 tube
does it. For the 90, 45 mm tube needed. (or 25+14+7) >>
Correct me if I'm wrong, but I believe that for 1:1 the total extension must
equal the focal length. So for the 50mm macros, which have 25mm of extension
with the helicoid, you need to add 25mm extension to get 1:1. Using George's
other example, then the 90 must have 45mm of helicoid travel (I don't know for
sure if it does or not).
Paul Schings
Coventry, RI
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