It also removes the need to have separate viewfinders for different lens focal lengths. Consider a beam traveling through a thin lens with focal length "f". Camera lenses are made in a wide range of focal lengths. Criteria relating CoC to the lens focal length have also been used. It can also be separated into focal or diffuse. This forces the observer to begin by selecting a focal point for focus. Longitudinal aberration is typical at long focal lengths. Transverse aberration is typical at short focal lengths. Zoom lenses are often described by the ratio of their longest to shortest focal lengths. One has a reticle graduated to allow measuring distances in the focal plane. Imaging studies in humans with focal dystonia have confirmed this finding. Variation in focal length is one of the chief benefits. Gelastic seizures can be due to focal lesions to the hypothalamus. These risks are greater in those with generalized rather than focal seizures. This relies on the exploitation of chromatic aberration (wavelength-dependence of focal length).