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Old 30th June 2009, 08:16 PM
Sandeha Lynch's Avatar
Sandeha Lynch Sandeha Lynch is offline
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Location: Wales (UK)
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I haven't used tubes for a while but roughly speaking if you double the focal length of the lens you increase exposure by two stops.

There are formulae and calculators on the web for large format users - you might search for bellows extension since the principle is the same.
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