Present-day 50-mm "normal" lenses are almost always the double Gauss type (Figure 7.12, middle column), derived from the Zeiss Planar of 1892. Back in the 1800s, several experimenters discovered that if you put two Gauss achromats back-to-back, each cancels out the distortion introduced by the other. The same thing was tried with meniscus lenses and other kinds of achromats, resulting in various "rectilinear" lens designs.
Double Gauss designs were not practical until the advent of anti-reflection lens coatings in the 1950s; before then, their sharply curved air-to-glass surfaces led to internal reflections. Previously, the standard camera lens had usually been a variant of the Zeiss Tessar, which is classified as a triplet derivative but was actually invented as a simplified Planar.
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