A few days ago I posted about using flash to freeze movement, and yesterday’s post talked about using a slow shutter speed to show motion. You might think these two concepts are mutually exclusive, but they can be combined to create a very dynamic image. And if you read those previous posts, you already know how I did this.
I’ll talk more tomorrow about the lighting set-up I used for this photo, but all you need to know for now is that I am using two flashes, plus a good deal of ambient light, so you could call it a three light setup. The flashes are off camera and behind me, one to my left, one to my right. The setting is the dancefloor at a wedding reception in Galway.
The light levels in the room were reasonably low, requiring exposures of about 1/15s at f/2.8 even when the ISO was bumped up to 800. At those shutter speeds, it would be very hard to get sharp photos, especially on a dance floor where the subjects were moving continuously.
Introducing flash into the equation allows me, if I want, to bring the shutter speed back to a reasonable level, or to use a narrow aperture and get a larger depth of field, and so keep more of the shot in focus. My first attempt at this shot did just that, by bumping up the flash power, and setting the shutter speed to 1/250s. The result is not good. The subjects are lit entirely by flash – they are pin sharp which is good – but they are against a dark background and the whole mood of the scene is lost. There is no sense of environment, and the whole effect is far too harsh. What the flash does do though is freeze the movement of the dancers, and so when I go to improve the shot, that is one effect I want to keep. So while the flash brings something to the picture, I need to use it in a way that allows the environment be shown also. I want to balance the light from my flashes with the ambient light in the room.
So I do two things.
Firstly, I reduce my shutter speed. I don’t want the background to be properly exposed as that is not to be the focus of the picture – but I do want to let it into the picture again, so I underexpose it, but not too much. To do this I end up with a slow-ish shutter speed of 1/30s. This technique, of controlling the ambient exposure when using flash to light the foreground, is called dragging the shutter. Because the flash is predominantly lighting the subjects, I can use the shutter speed as an independent control over the background exposure without having a major effect on the foreground exposure.
Secondly, I adjust the flash power down a bit, because at 1/30s, the foreground too will be only a little underexposed now, and so needs less light to bring it back to a proper exposure than it did with a faster shutter speed.
So with the settings changed, I fire off another shot. Now both that slow shutter speed and the flash are doing their part to create the shot you see above. The flash, which fires in a thousandth of a second or so, and is the main light for the subjects, freezes them and gives me a nice sharp shot of their faces. The shutter stays open for a long time (relative to the flash) and allows some of the ambient light in the room in to the sensor. This brings up the background, and a little more of the subjects, blurring the edges, and enhancing that sense of motion.
The end result captures the dancers in full flight as they swing around the dance floor, and shows the environment in which they are in, giving the viewer a context for the shot. Incidentally, the flash in this shot fires at the end of the exposure, just before the shutter closes, and not at the start. There’s is good reason for this, but that’s a story for another day.
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