<P> When the POF is rotated, the near and far limits of DOF are no longer parallel; the DOF becomes wedge - shaped, with the apex of the wedge nearest the camera (Merklinger 1993, 31--32; Tillmanns 1997, 71). With tilt, the height of the DOF increases with distance from the camera; with swing, the width of the DOF increases with distance . </P> <P> In some cases, rotating the POF can better fit the DOF to the scene, and achieve the required sharpness at a smaller f - number . Alternatively, rotating the POF, in combination with a small f - number, can minimize the part of an image that is within the DOF . </P> <P> For a given subject framing and camera position, the DOF is controlled by the lens aperture diameter, which is usually specified as the f - number, the ratio of lens focal length to aperture diameter . Reducing the aperture diameter (increasing the f - number) increases the DOF because the circle of confusion is shrunk directly and indirectly by reducing the light hitting the outside of the lens which is focused to a different point than light hitting the inside of the lens due to spherical aberration caused by the construction of the lens; however, it also reduces the amount of light transmitted, and increases diffraction, placing a practical limit on the extent to which DOF can be increased by reducing the aperture diameter . </P> <P> Motion pictures make only limited use of this control; to produce a consistent image quality from shot to shot, cinematographers usually choose a single aperture setting for interiors and another for exteriors, and adjust exposure through the use of camera filters or light levels . Aperture settings are adjusted more frequently in still photography, where variations in depth of field are used to produce a variety of special effects . </P>

To control depth of field and amount of light what should be adjusted