As others here have said, approach slowly, and if possible, photograph them in the morning. If mornings aren’t avalaible, overcast days are fine too. From my experience, robberflies tend to be more cooperative than other flies, though depending on where you live, they might not be present. As for photos, if you’re using a phone, you might want to buy a clip-on macro lens, to get the maximum quality. Most flies will fly away as soon as they see the camera, but some will allow decent photos. Mostly, it just requires a lot of practice, patience, and a little luck. Here are some photos I’ve taken of flies:
