Anyone who has ever gazed at one of Thomas Struth's photographs of museumgoers looking at masterpieces knows that in grandeur and intensity they hold up to the old masters. That makes sense: before becoming a prime representative of the new German photography, Struth studied with painter Gerhard Richter, whose canvases propose an alliance between painterly technique and photographic vision. Struth's first U.S. retrospective includes nearly 100 works: from the small black-and-white street scenes of the seventies and portraits of the eighties to the enormous cityscapes and landscapes of the nineties, for which he traveled as far as the Yangtze and the jungles of Brazil. ("Thomas Struth," Dallas Museum of Art; May 12—August 18). • Matisse and Picasso's artistic rivalry began when they met at Gertrude Stein's salon in 1906, but each ended up regarding the other as his only true peer. After a show that comprises some 30 groupings of paintings and sculpture, you'll understand why. ("Matisse and Picasso," Tate Modern, London; May 11—August 18).