How much bellows extension do you need to focus to 1:1 with a given size lens, 120mm lens on 6x9 format camera for example. Is there a simple formula?

-- Hank Graber (hgraber@narrativerooms.com), January 18, 2002

Answers

You will need twice the focal length, e.g., a 120mm lens will need 240mm of bellows ("extension") to achieve 1:1 magnification. This is independent of format. The formula is (M+1)*F, where M=magnification (0=infinity focus,no magnification, .5 = 1:2, 1 = 1:1, etc.) and F is the focal length.

-- Steve Baggett (sbaggett@midsouth.rr..com), January 18, 2002.

Steve is right about the bellows draw except if you are using a telephoto design lens, where the bellows draw is shorter due to the design of the lens. But this is a special case, as most lenses that are used with large format cameras are of a standard (fairly symmetrical) design.

-- Björn Nilsson (b.w.nilsson@telia.com), January 19, 2002.