In this type of lens, the lens group behind the diaphragm has nearly the same configuration and shape as the lens group in front of the diaphragm. Symmetrical lenses are further classified into various types such as the Gauss type, triplet type, Tessar type, Topogon type and orthometer type. Of these, the Gauss type and its derivations is the most typical configuration used today becauseits symmetrical design allows well-balanced correction of all types of aberrations, and a comparatively long back focus can be achieved. The Canon 50mm f/1.8 released back in 1951 succeeded in eliminating the comatic aberration which was the sole weak point of Gauss type lenses of that day, and thus became famous as a historical landmark lens due to the remarkable improvement in performance it afforded. Canon still uses a Gauss type construction in current lenses such as the EF 50mm f/1.8 II, EF 50mm f/1.0L USM, EF 50mm f/1.4 USM and EF 85mm f/1.2L USM. The Tessar and triplet type symmetrical configurations are commonly used today in compact cameras equipped with single focal length lenses.canonglossaryeu
Lights placed at more or less equal heights and equal-but-opposite angles and set at equal intensities. Because such lighting violates the principal of Dominance, it looks unnatural, confusing, or boring.