The German photographer and collector Thomas Walther started purchasing photographs in the 1970s, just after he moved to America. Walther focused on experimental European and Modernist work from the late 1800s into the 20th century. The list of photographers in his collection is a Who’s Who of the art form. He bought from Claude Cahun, El Lissitzky, Edward Weston, André Kertész, to name four. He also frequented flea markets, buying snapshots that took his fancy. Walther’s magisterial collection includes urban landscapes, portraits, nudes, photojournalism, and wild experiments. It is adventurous and sensitive. This exhibition puts 230 masterworks on view, chosen from the 350 photographs Walther gave to MoMA. It’s a portrait of energized seeing. —E.C.