We stayed in a treehouse near Habarana, Sri Lanka. Since the treehouse had no walls, insects were free to enter. One night after going to bed, we noticed blinking and realized that fireflies were circling above us. We got up, attached the camera to a tabletop tripod, and did a few test shots at ISO 6400 to determine exposure settings (one second at ISO 6400 is equivalent to one minute at ISO 100). Based on the test shots, we used an intervalometer to take a sequential series of two-minute images. A sequence of seven of these images were merged using the Lighten blending mode in Photoshop to make the equivalent of a 14-minute exposure. The following photo is the result.

The images were captured on February 3, 2018. Specs for each of the seven images are:

Canon 100D, ISO 100, f/2.8, 120 sec, 17 mm.

As a final note, the image is not as sharp as I would like it to be.  I wasn’t planning to take this shot and I was afraid that using any light would drive the fireflies away. The next evening, I set the camera up before dark and was ready. The result was a series of extremely sharp — but firefly free — images of the ceiling.