In sun printing, a process invented in the 1830s, natural sunlight is used to produce an image on plain paper treated with minerals without a darkroom. Linda McCartney began experimenting with this technique early on, and the results are some of her finest photographs. Combining the spontaneity of modern photography with the subtlety of old printing methods, here are richly-hued portraits of family and friends as well as moody, deeply-shadowed landscapes...