Imaging performance of aspherics in comparison with spherical surfaces.
The article compares rotational-symmetrical aspherics with spherical surfaces with regard to their expected imaging performance. Under simplifying assumptions, formulas are derived and numerical values obtained, e.g., of the ratio R of the number of spherical surfaces to that of aspherics which should give the same imaging performance. R is a measure of the expected efficiency of applying aspherics. In most cases 1 < R less, similar 4 but in limiting cases, R ? infinity. These limiting cases correspond to very small field angles and wide apertures (strictly aplanatic systems) and to very small apertures and wide field angles. For medium field angles and apertures, R gradually increases with an increasing number of optical surfaces of the system.
Schulz
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