I just wanted to throw this one out there, because I liked the effect. It was taken four years ago as an experiment, and came out differently than expected. Take a moment and see if you can figure out how it was produced.
I can provide a clue: Most times, TTL flashes operate by measuring the light that makes it to the exposure meter within the camera, and gets shut off when enough is detected. This happens remarkably fast, in a few ten-thousands of a second, so while the flash may look instantaneous to us, it is actually started, then halted by the camera when it determines enough light has been received by the film or (in this case) digital sensor. Except, when the subject is dark and insufficient light is being received, the flash can actually discharge the entire capacitor, which results in some light fading at the end of the exposure. Therefore, moving objects appear brightly lit at the beginning of the exposure, but get dimmer towards the end. This can result in streaking, with the apparent direction opposite of the actual.
Not enough? How about if I tell you I was aiming straight up?
If you haven’t gotten it with those, I’ll simply tell you I stuck the camera out from under the edge of the roof during a downpour and fired off a frame with the flash, which illuminated the rain. The closest drops showed the greatest apparent motion, appearing to be moving towards the center of the frame by the fading flash, when in actuality they were falling past the wide-angle lens. Some drops are well out of focus, others not so. The color effects, such as the red spheres to the left side, I haven’t fully explained, but may be illuminated by light passing through water droplets on the flash head, which was refracted into different colors of the spectrum. I would have suspected a more uniform effect among close drops, though, so maybe it’s the flash light refracting through drops suspended in the air, illuminating close neighbors.
You can click on the image for a slightly larger version. I haven’t done any editing on this at all except for resizing – no color enhancements or contrast changes. If you noticed some “dead patches” in the frame, these were most likely caused by water drops already on the lens, blurring the drops beyond distinction. Just a neat effect, and I was impressed with the amount of color that showed up.