In the long corridor leading to Nude Visions—a fantastic look at the 150-year history of nude photography at Hamburg’s Kunst und Gewerbe museum—a notice reads: “A stare is an erection of the eye.” In another setting, we’d be prepared for an orgy, but here we’re pretty sure it has a more academic meaning. At a time when the Internet has turned the world into gynecologists, to borrow loosely from Ab Fab, it’s fascinating to see how notions of nudity have changed since the 19th century, when the nascent art form represented bodies almost scientifically. This changed in the 20th century, thanks to exoticism, New Objectivity, surrealism, Guglielmo Plüschow’s homoerotic photographs and latter-day provocateurs including Helmut Newton. The exhibit ends with the realistic and raw imagery of contemporary stars Juergen Teller and Nan Goldin. But most haunting are two images from the 1970s: an extraordinary picture by Charles Gatewood of a woman masturbating at a cocktail party and another by Tim Ulrichs of men and women having sex in front of famous paintings. Not eye sex, but sex sex.