triple extension

Definition: (1) Mechanical arrangement that stacks and telescopes three focusing rails, one on top of the other, to permit extra-long extension in a relatively compact design. * Used primarily in field-cameras where long extension is required for close-up photography and use of long-focal length lenses but the focusing rail must be small enough to be folded up. (2) Bellows design permitting extension of three or more times the focal length of a normal lens for the format e.g. an extension of 450mm for 5x4" format.

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