A little photographic trick:  Look for opportunities where the fly-caster has a dark background behind him, or at least a patch of dark, across which his fly line will pass when he casts. You might have to ask him to step forward out of the shadows just a little in order to get the sunlight to catch his arcing line. Then take pictures on continuous shooting , in order to get the line at the perfect spot. These opportunities will present themselves more in the early morning or late afternoon, and more so in steep river canyons, where shaded vertical