For a spherically curved mirror in air, the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two. The focal length is positive for a concave mirror, and negative for a convex mirror. In the sign convention used in optical design, a concave mirror has negative radius of curvature, so  where "R" is the radius of curvature of the mirror's surface. See Radius of curvature (optics) for more information on the sign convention for radius of curvature used here. Camera lens focal lengths are usually specified in millimetres (mm), but some older lenses are marked in centimetres (cm) or inches. Focal length ("f") and field of view (FOV) of a lens are inversely proportional. For a standard rectilinear lens, FOV = 2 arctan , where "x" is the diagonal of the film. When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane. To render closer objects in sharp focus, the lens must be adjusted to increase the distance between the rear nodal point and the film, to put the film at the image plane. The focal length ("f"), the distance from the front nodal point to the object to photograph ("s"1), and the distance from the rear nodal point to the image plane ("s"2) are then related by:  As "s"1 is decreased, "s"2 must be increased. For example, consider a normal lens for a 35 mm camera with a focal length of "f" = 50 mm. To focus a distant object ("s"1 ≈ ∞), the rear nodal point of the lens must be located a distance "s"2 = 50 mm from the image plane. To focus an object 1 m away ("s"1 = 1,000 mm), the lens must be moved 2.6 mm farther away from the image plane, to "s"2 = 52.6 mm. The focal length of a lens determines the magnification at which it images distant objects. It is equal to the distance between the image plane and a pinhole that images distant objects the same size as the lens in question.