Thursday, July 10, 2014

Whirling Disks and Rear Curtain Sync

Rear curtain flash sync is a function on many cameras. The way it works is that instead of triggering your flash when your shutter opens (generally for a much longer time than the length of the flash light pulse itself), it opens the shutter, then triggers the flash right before the shutter closes.
Why would you want this? Well, if you're using a slow shutter speed, you'll get motion blur or light trails from anything that's moving fast. With normal (front curtain) sync, the flash goes off, the shutter stays open, and you get trails that are made after the flash. With rear curtain synch, the trails are made first, then you get flash and light the rest of the scene at the end of the exposure. The classic example is a moving car, where you get an image of a car that's already gone by you, with fantastic blur behind it. Using front curtain sync, you'd get the car before it passes by, and light streak in front of the car, not nearly as cool.
Well, I find it's pretty difficult to find applications other than moving cars for this cool technique. At the River and Roots music festival, I found one. These guys were playing what I would normally call "Frisbee," but with a disk that has its own light in it. They were playing in the dark. I thought, hmm. I bet that disk would make a great light streak. So, I tried it. Amazing! It made this really cool nested helix shape as it flew by. Next step, try rear curtain sync to see if I could capture the streaks and the player. Here's how that looks:
I like this shot a lot, but I wanted the streaks to be longer, and I didn't like the distracting lights in the background (at left).
First, the background lights. One of the other players was near the river, which was completely dark. Goodbye background distractions:
Getting the timing just right was tough, but I was getting the hang of it.
So, how to lengthen the light trails? Longer exposure, of course. So, I went to a full second exposure, with the flash at the end. Since the ambient scene was so dark, I didn't even have to change my f-stop or flash settings. This is exactly the effect I was looking for:
Whirling Disk, by Reed A. George
Nikon D700, Nikkor AF-D 50mm f1.4 Lens
iso 1600, f4, 1 second exposure, rear curtain sync flash
Great fun. I don't think any other photographer at the festival captured a shot just like this.