Final answer:
Lewis Hine is the artist best known for his use of photography in early social reform, particularly through his work with the National Child Labor Committee and the Pittsburgh Survey. Alfred Stieglitz also played a crucial role in elevating photography as a fine art with his leadership in the Photo-Secession movement.
Step-by-step explanation:
The artist best known for being an early leader in the use of photography for social change is Lewis Hine. Hine utilized photography to spotlight social issues such as poor working conditions and child labor.
His work with the National Child Labor Committee and contributions like the Pittsburgh Survey were instrumental in raising awareness and influencing social policy during the early 20th century. Also noteworthy in the realm of photography as an art form was Alfred Stieglitz, who led the Photo-Secession movement and was instrumental in promoting photography as a fine art.
His work, The Steerage (1907), is a prime example of his efforts in achieving recognition for photography as an art equal to painting and sculpture.