Early Minox cameras from Minox A/III to Minox B were equipped with a four-element, three-group Complan (lens) designed by ex-Leica lens designer Arthur Seibert. The Complan lens has a curved film plane, hence in these cameras the negative must be held in an arc to improve the edge-to-edge sharpness of the image. The Minox enlarger also holds the negative in this same curve. Later models, beginning with late model Minox B, to the current model TLX, using the 15 mm f/3.5 four-element, three-group flat-field Minox lens, holds the negative flat. The advance was attributed by Rolf Kasemeier (Small MINOX Big Pictures 1971 edition) to new rare-earth element, high-index, low-dispersion, optical glasses becoming available (probably from Schott Glass, of Jena). Note that lens performance between old and new complan/minox lenses was rated by MINOX themselves as identical.