Telescopic optical components that have been used as the unit cell of two-dimensional arrays, and which therefore form homogeneous METATOYs, include a pair of identical lenses ( focal length f ) that share the same optical axis ( perpendicular to the METATOY ) and that are separated by 2 f, that is they share one focal plane ( a special case of a refracting telescope with angular magnification-1 ); a pair of non-identical lenses ( focal lengths f _ 1 and f _ 2 ) that share the same optical axis ( again perpendicular to the METATOY ) and that are separated by f _ 1 + f _ 2, that is they again share one focal plane ( a generalization of the former case, a refracting telescope with any angular magnification ); a pair of non-identical lenses ( focal lengths f _ 1 and f _ 2 ) that share one focal plane, that is, they share the direction of the optical axis, which is not necessarily perpendicular to the METATOY, and they are separated by f _ 1 + f _ 2 ( a generalization of the former case ); a prism; and a Dove prism
