A Different Approach
No pictures in this one…
In my last blog, I talked through the whole process of how I lit a particular shot. The most challenging aspect of it was the balancing of flash to the ever changing ambient light, so as to maintain the main (ambient) to flash (fill) ratio. My approach involved fixing the shutter speed and ISO on the camera and then using aperture to dial up and down the exposure as the ambient light shifted. Unfortunately this effected not just the ambient part of the exposure, but also the flash part too, which was an undesired side effect. However I compensated for this by changing the flash power to match the changes in ambient.
The day after I wrote this, I was working my way through the various photography blogs that I follow and was very interested to read David Hobby’s (strobist) technique for automatically matching flash to ambient, which was simplicity itself and to a large extent would have allowed me take exactly the same image, with far less effort.
Rather than go the approach of fixing ISO and shutter speed, which is the one I took; he suggested that you actually do the opposite Your flash is put in manual mode and set to a fixed output value as per an initial metered reading. The flash output now can never change.
On the camera body, ISO is also fixed as per my previous method. But the big difference is that where previously I fixed shutter and varied aperture, in this approach you fix aperture and vary shutter.
As the ambient light changes, you can increase or decrease shutter speed accordingly. If it gets brighter, we can increase shutter speed to compensate and as it gets darker, we can use longer shutter speeds; each time keeping the ambient light exposure at the same level.
The genius thing about this approach is down to the fact that shutter speed has no effect on the flash part of the exposure. So as we change the shutter speed to try and maintain the same level of ambient light, the flash part of the exposure never changes an entirely more elegant solution and far less work.
There are some caveats however Increasing and indeed decreasing your shutter speed can have differing effects on your image. You need to ensure your shutter speed is sufficiently fast so as to guarantee no camera shake (using a tripod can help) and no subject movement. It also can’t be so fast so as to go beyond the maximum flash sync speed.
Using the setup I had, where I was hand holding a long lens (100mm), this would imply I could probably drop to about 1/50 or maybe a little lower. On the faster end, I would be limited to a speed of 1/250 by the flash sync speed. So all in all, I’d have about 3 stops to play with.
As it turns out, on the day of the shoot, this wouldn’t have been enough because the difference between direct sun light and when it went behind the clouds, was far greater than 3 stops. But it’s a very interesting technique that I will certainly try in the future.