He's taken pictures of everyone from Angelina Jolie and Donald Trump, to female bodybuilders and the homeless of Los Angeles. Ahead of his new exhibitions at Galerie A and at Paris Photo festival, one of the world's most celebrated portrait photographers, Martin Schoeller, speaks to Eve Jackson about whether you can sum up a person in one photo, the difference between stars and the non-famous and who he preferred out of Clinton, Obama and Trump.