Alfred Stieglitz, 1922-35 series. Cloud studies as the first 'abstract' photography — proved photographs could express feeling without subject.
From Wikipedia
Equivalents is a series of photographs of clouds taken by Alfred Stieglitz from 1925 to 1934. They are generally recognized as the first photographs intended to free the subject matter from literal interpretation, and, as such, are some of the first completely abstract photographic works of art.