Alfred Stieglitz was an American photographer who played an instrumental role in the consideration of photography as an art form in its early years. (elaborate)
Of his works, the series that really stood out to me was Equivalents, his photographs of clouds, largely regarded as the first set of abstract photos. These photos were early attempts to capture the skies in a photograph, and were influenced by his previous attempts where the limitations of photography at the time stopped him from achieving the desired effect of the photograph. A recurring theme throughout the photographs is the absence of any features that show a flat plane, stopping the viewer from working out where up or down is, and where the photograph was taken.