Bellows Draw

[Hank Graber]

How much bellows extension do you need to focus to 1:1 with a given size lens, 1 20mm lens on 6x9 format camera for example. Is there a simple formula?

[Steve Baggett]

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.

[Bjorn Nilsson]

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.