William Wegman American, b. 1943

Two Weimaraners sit side by side on a warm brown studio ground, front legs straight, chests out, in the posture dogs adopt when somebody has told them to. Above the head of the one on the left, floating, an orange ball. Both dogs have their eyes turned up and to one side, following it. Neither is looking at us, and the ball is the only thing in the picture with a cool colour in it.

That ball is the reason the photograph exists. A 20 by 24 Polaroid needs a long enough pause to be exposed and there is no way of explaining that to an animal, so somebody stood off camera and held something up. What the print records is not obedience. It is attention: two heads locked onto the same small object, which is the only reliable way to get two dogs to hold still at once.

Everything else is Wegman's studio at its plainest. No costume, no props, one sienna ground, two dogs in their own skin. The Polaroid has laid its warm brown over all of it so that the animals, the floor and the air behind them are very nearly the same colour. Take the ball away and this is a double portrait. Leave it in and it becomes a photograph of the moment before somebody throws.