24 December 2014

I recently took some photos of my kids’ end-of-year concert with a hot-shoe flash. The results were disappointing in terms of freezing motion; an otherwise lovely image of my daughter at the curtain call was spoiled by motion-blur of her hand as she waved. Flash supposedly freezes motion, so how could this be? I investigated.

I was using a Canon Powershot G15 with a Speedlite 270EX II in the hot-shoe. Here’s what I understand about its metering in Av and Tv modes with an external TTL flash.

At my kids’ concert, I was shooting in Av mode at f/2.8 (the maximum at the selected effective focal length of about 100 mm), with TTL metering and ISO 400. I’d chosen this aperture and ISO to reduce the flash recharge time. I had “Slow Synchro” set to OFF and have not selected “High Speed Sync”. I wasn’t bouncing the flash.

I was about 10 meters away from the stage, which means I’d need a guide number of about 14 meters. (The required guide number is the distance multiplied by the f-number multiplied by 10 and divided by the square root of the ISO, and 10 meters × 2.8 × 10 ÷ √400 is 14 meters.) The Speedlite 270EX II has a maximum guide number of 27 meters, so I’m within range and would probably need only about 1/4 power.

The camera selected a shutter speed of 1/100 second. That suggests that the contribution from ambient light was sufficient for a good exposure; if it had been too faint, the camera would have selected 1/60 second. Thus, the contribution from ambient light overwhelmed the flash, and instead of freezing motion with a 1/1000 second flash, I was effectively taking an ambient-light exposure with a shutter speed of 1/100 second. No wonder I saw motion blur. As further confirmation, the photos don’t show the sharp shadows or red-eye that I would have expected from an unmodified on-camera flash.

How could I have know this was going to be a problem? By seeing how the camera was setting itself up for the flash exposures, and taking an ambient light exposure with the same settings but no flash. Or, more quickly, by seeing how the camera was setting itself up for flash exposures and for ambient exposures.

How could I have fixed this problem? My best options were:

The advantages of these two changes is that they would have reduced the ambient contribution without requiring more flash energy. In other circumstances, I’d probably prefer using the exposure compensation dial, since it would allow me fix the aperture to control depth of field.

Other options include:

The disadvantages of these is that they require more flash energy, but I had two stops of energy to spare.

Two manual options were:

These four options would not have worked very well in this situation, but might work better elsewhere:

None of this is rocket science. All I needed was to have had a better understanding of my camera and to have remembered to take an exposure without flash to check the ambient light. Next time.

© 2014 Alan Watson Forster.