There’s no rule that you have to paint only landscapes or cityscapes when you’re outdoors with your oil kit. You can paint people, too.
French academic painter Jehan-Georges Vibert executed this 9x14 inch oil study called “At the Corrida” outdoors at a sporting event in 1875. Evidently he painted each figure spontaneously as they struck momentary poses.
Click to enlarge. There's a man leaning over the railing, cheering, and three different women holding up their fans to shield their faces from the sun. Each is a study in character, each sketched from candid subjects.
The piece shows signs of being painted in a slotted box, a typical plein air set-up from the time. The study was probably finished out to the edges later.
See 29 images by Vibert at Art Renewal Center, link.