Singlets are available in a wide variety of shapes. Plano-convex and biconvex lenses have a positive focal length, plano-concave and biconcave lenses have a negative focal length. Meniscus (convex-concave) lenses may be either positive or negative and are used when very long focal lengths are required. Aspheric lenses have one aspherical surface and one spherical or plano surface. They are used principally as light condensers. Spherical ball lenses are ideal for fiber-to-fiber and diode laser-to-fiber coupling. Gradient-index (GRIN) lenses combine refraction at the surfaces with continuous refraction throughout the thickness of the optic.