A portrait of the painter Georgia O'Keeffe by Alfred Stieglitz is to lead a photography sale at Christie's New York on March 31 with a $400,000-600,000 estimate.

The lot dates to 1918, a time when Stieglitz was in his mid 50s and had established himself as one of the most influential figures in contemporary American art.

Stieglitz and O'Keeffe married in 1924

He was behind the photo-secessionist movement of the early 1900s, which had been key to establishing photography as an art form.

Sarah Greenough explains in her book Alfred Stieglitz: The Key Set: "Almost as soon as O'Keeffe arrived in New York from Texas on 9 June 1918 Stieglitz began photographing her with what she later described as 'a kind of heat and excitement.

"As his studies of her evolved, they became less about O'Keeffe as an artist and more about her as a woman and lover, and of the sexual passion they shared…

"He photographed her like the lover that he was, entranced with every inch of his beloved, but also studied her like a painter or sculptor analyzing every inch of a model's body to understand how to portray the whole more accurately, as, for example, Rodin did."

They married six years later, in 1924.

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