Double extension is a feature of large format cameras that allows the bellows to be extended to approximately twice the focal length of the lens in use. This extended bellows position enables the camera to focus on subjects at very close distances.
Photographers use double extension primarily for close-up and macro photography, where precise focusing and increased lens-to-film distance are necessary to capture fine details. This capability provides flexibility in composing and magnifying small subjects while maintaining image sharpness and perspective control.