The plein air painter has no such box. Whatever his subject, it sits in a world that writhes with distractions. Another interesting tree-shape, a building with a curious door, a colorful swath of meadow -- all things he didn't see at first, and all of them just outside his chosen frame -- compete for a place in the painting.
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Where I live, which is by the broad ocean with breathtaking vistas of sea cliffs, I am often tempted to take in an extended range worthy of Albert Bierstadt. But when I set up my pochade box and my usual 8x10 panel, experience tells me to settle for less. If my first choice was a stunted, storm-blasted tree jutting up out of a field of wild roses, that is what I paint. I forgo the scenic piles of driftwood just outside my chosen view, and the scallop boats trawling the bay, and the waves crashing on the most distant cliffs, and -- but you get the point.
I tell myself that life is long, and I will eventually paint all those other scenes. This is not true, of course, but it does take the pressure off.