The Scheimpflug principle describes what happens when the lens is tilted relative to the film or sensor. If the subject plane, the lens plane and the image plane all meet along one line, the whole subject plane is in sharp focus. It is named after Theodor Scheimpflug (1865-1911), an Austrian army officer who patented it in 1904, though Jules Carpentier described it earlier.
It is used with view cameras and tilt-shift lenses. In a landscape, tilting the lens forward can make the plane of focus run along the ground. Near and far parts can then be sharp at a wider aperture. Tilt does not add depth of field. It changes its angle.
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