Recently a friend and I were discussing "en plein air" painting. I wasn't surprised to realize she had some confusion in her mind regarding Impressionism and painting on location. I think she considered them one and the same. Part of the confusion stems from the narrow perception that Impressionism has only to do with landscape painting. And though Impressionism (as a style) is most associated with landscapes, it took the early impressionists only a short time to begin seeing other things from an impressionistic mindset; and to begin applying the style to other subject matter; to the point that very quickly, nearly all subject were "fair game."
It's little wonder people are confused relative to the terms Impressionism and "plein air." While it's true that Impressionism evolved out of plein air painting, the act of painting out-of-doors goes back well before Monet's "Impression, Sunrise," at least as far back as the Barbizon school in the mid 1850s (Barbizon is a small French town in the Fountainbleu Forrest). Now, having said that, plein air painting could, I suppose, be almost ANY style, though Impressionism, with it's quick renderings (sketches in oils) especially lends itself to this method of painting on location. There are purists would insist that all Impressionist paintings be started and, most of all, FINISHED out-of-doors. But that was often not the case then and seldom the case now. One reason for this was that in it's "heyday," the Impressionists were still feeling their way--learning. And that could ONLY be done out-of-doors. But now, the precepts of divided brush strokes, eye-blending of colors, prismatic, perceptual color, atmospheric renderings, aerial perspective, and so on, is all pretty academic and well-understood by those interested in this style of painting.
So, must all Impressionism be painted "en plein air?" The answer to that is a somewhat qualified, no. The qualification of that negative response is that, in trying to capture momentary light, Impressionism is probably easier and BETTER when done on location, though I suppose one could paint an Impressionist scene through a window (which Monet, among others did). And, I suppose, a really experienced Impressionist could work from photos (a stretch) or memory, perhaps even imagination, though by the time you reach that far you are risking a transition into EXpressionism. As usual, when it comes to lines dividing one "ism" from another, they often get rather blurry when you go bending the customary "rules." But again, keep in mind, Impressionism is not and WAS not limited to landscapes. Subjects such as figures, still-lifes, portraits, and flowers, to name just a few, were also painted using an Impressionist style, and while some these undoubtedly WERE painted "en plein air", it's likely the majority were NOT.