RoboRoach

Nikon D7000, 55mm (18-55), 1/2, f/13, ISO 100, cropped

Once a semester I like to devote a class period to useless photo techniques. To be sure, these tricks have their uses. But in Intro to Photography I put a premium on pictures that capture perceptions. For me, saving a moment in time is the ultimate photographic experience. Elaborate set-ups and “fake” photos don’t do as much for me. Thus I don’t generally use techniques that require a lot of prep work.

Still, some of them are fun to play with. This semester I decided to try one I’d never done before: rear curtain flash. This technique combines a long exposure with a flash to create a double exposure without actually exposing the frame twice.

To get this effect, start by slowing the shutter way down (I’m using a half second exposure here), which of course requires a tripod. A rapidly-moving object such as this small robot – dubbed RoboRoach by one of my students – will move a lot in half a second, resulting in a photo that looks like this:

Nikon D7000, 55mm (18-55), 1/2, f/13, ISO 100, cropped

In order to get the subject itself to actually show up, I need a shorter exposure as well. Normally that means a short shutter speed, but here that isn’t an option. However, I can fake it by firing the flash right at the end of the exposure. In the photo at the top of this entry, you see the extreme motion blur of the half second shutter combined with a flash that lasts for a fraction of a second at the end of the shot. The flash freezes RoboRoach, combining the clear, frozen shot with the blurry “vapor trail” of light reflected from the shiny spots.

The trick is to fire the flash at the very end of the shot, which is called “rear curtain flash.” Normally cameras fire the flash at the front end of the exposure, based on the assumption that what you want is what you’re looking at exactly when you press the shutter button, not at the end of the exposure. In truth, most flash photography is done at higher shutter speeds (use 125 as a default), so the “curtain” doesn’t matter much.

But to create this effect, I want the streaky blur to trail out behind RoboRoach. If the flash fires at the start of the shot, the vapor trail will extend out in front of the subject. Thus I need a camera with a Rear Curtain Flash setting, which my D7000 just happens to have.