The photo above was taken with a Sigma 105mm f/2.8 Macro lens. The camera was set to Aperture Priority exposure mode and a shutter speed of 1/60s was chosen by the camera at ISO 100. To obtain the best possible image, I mounted the camera on a tripod and used a cable release to minimize blurring caused by camera shake. The photo came out nicely exposed and with decent sharpness.
I then tried to expand the depth of field (DOF) more by reducing the size of the aperture. At f/16 and 1/4s, I had a problem. The photo came out blurry like the following. I tried my best to avoid camera shake, but I could do little to prevent the wind from rocking the flowers!
I could have waited for the wind to stop or just kept clicking away and hoped I was lucky to catch a break from the wind. I could also increase the ISO to increase the shutter speed. A simple calculation indicated that I needed ISO1600 to get a shutter speed of 1/60s. That’s probably not fast enough to stop the motion but high enough to degrade the image quality quite a bit. The Nikon D200 I was using isn’t known for its good high ISO performance.
A solution quickly came to mind. I mounted a SB-600 on the camera, pulled out the built-in wide-flash adapter in an attempt to soften the light slightly, and took a shot in TTL mode at f/16 and 1/60s. Now the blurriness is completely gone. With shutter speed of 1/60s, the ambient was 4 stops under exposed and the SB-600 beaome the main light source. With typical flash duration between ~1/1000s and 1/25000s dependent on output level, the motion of the subject is frozen by the flash pulse.
Of course, mounting flash directly on the camera isn’t an optimal lighting setup for macro photography. The photo, despite being sharp and having wider DOF, has an obvious flash look, with distracting shadows. I could have played with it a little more if I wasn’t called into the house for dinner. Even so, I still found it useful to illustrate an important concept of flash photography: flash pulse duration vs. flash output level.
Unlike a household light with a dimer, the flashes for photography adjust output level mostly by changing its light output duration rather than its output amplitude. For example, the Nikon SB-600 has the following output vs. pulse duration relationship:
- 1/900th sec. at M 1/1 (full) output
- 1/1600 sec. at M 1/2 output
- 1/3400 sec. at M 1/4 output
- 1/6600 sec. at M 1/8 output
- 1/11100 sec. at M 1/16 output
- 1/20000 sec. at M 1/32 output
- 1/25000 sec. at M 1/64 output
Even at full power, the flash pulse is quite short, much shorter than typical shutter sync speed. That’s the reason shutter speed doesn’t affect flash exposure but only the ambient exposure. By making the flash dominant light source, you can use the very short flash pulse to freeze motion.
Keywords: Flash photography, Flash Tips