Constable depicted a lock keeper in a waistcoat and rolled shirtsleeves turning the hoisting bar of the lock gates with a wooden handspike. His dynamic pose is the focus of the scene. The skipper in the barge in the lock chamber holds a steadying rope around a bollard while waiting for the lock keeper to release the water. Water is already escaping from the lock and churns into the mill pool below. A boy with a dog beside him waits for the water level to drop to heave a chain to open the left lock gate. The boat can then pass out of Flatford Lock to the lower level of the River Stour and travel downstream. Behind the lock Constable depicted Dedham Vale across to the church tower. He divided the image equally between land and sky with a massive group of oak trees.
Lucas began working on the plate in February or March 1832 (Shirley 1930, p. 198). He based the image on one of Constable’s upright versions of the Lock, probably the painting now in a British private collection. This image differs from the horizontal versions of the subject where, instead of the barge being lowered to go downstream, in the horizontal versions a boat with a sail waits to be lifted to the upper level to proceed up river. And unlike the horizontal versions, here the trees on the far bank are oaks rather than willows.