 In the early to middle 20th century, American photographer Alfred Stieglitz (1864–1946) created a series of photographs of clouds, called "equivalents" (1925–1931).According to an essay on the series at the Phillips Collection website, "A symbolist aesthetic underlies these images, which became increasingly abstract equivalents of his own experiences, thoughts, and emotions".More recently, photographers such as Ralph Steiner, Robert Davies and Tzeli Hadjidimitriou (see catalogues listed below) have been noted for producing such images.