“To make a single photograph,” says the artist Idris Khan, “I use a lot of found imagery. A lot of photographs (the Houses of Parliament, Tower Bridge, St. Paul’s) were from the 1930s onward. I looked at a lot of stock photography as well, and went to all the horrible tourist shops in London to get every postcard I could find. I used 70 to 100 images for each picture. … I try to capture the essence of the building — something that’s been permanently imprinted in someone’s mind, like a memory."