Very long focus lenses may require that the camera be fitted with special extra-long rails and bellows. Very short focal length wide-angle lenses may require that the standards be closer together than a normal concertina-folded bellows allows. Such a situation requires a "bag bellows", a simple light-tight flexible bag. Recessed lensboards are also sometimes used to get the rear element of a wide angle lens close enough to the film plane; they may also be of use with telephoto lenses, since these compressed long-focus lenses may also have very small spacing between the back of the lens and the film plane.