A still from Unseen Skies, by Yaara Bou Melhem Courtesy of Participant Media A photograph of the sky by Trevor Paglen can look like a massive abstraction, except for a tiny speck, a surveillance drone, spotted like a malignant dot on a chest x-ray. His images of secluded military sites in Nevada can also ooze with colour from the churning heat and dust. In the new documentary film Unseen Skies, directed by Yaara Bou Melhem, Paglen calls the effect “impressionistic haze”. Photographing those places, often from miles away (or farther), is about “seeing and not...