Later in the evening, it was the photographer in the center who was waving a plate glass filter in the upper portion of the lens. He was doing time exposure shots. In the old days in dark room work, one would use a dodging tool under an enlarger. One of the photographers recorded the event with an iPhone (second from the right).
Finally, during my youth (at 17) I invented a contraption, a small rectangular box with two sliding doors in front, and hole for my mother's Voigtlander camera lens to fit in the back.I would take a photo with one door open, and take a second photo without advancing the film, with the other door open. I took a photo of "a photo myself and my imaginary twin". Of course, it was a double exposure. This is a scanned image of the ancient photo.