Flash Guns In the Sun
One way to give an exterior portrait a moody look is to use flash—even during the day, when the sun is still out. But how much power do you actually need to beat the sun at its own game? As a proof of concept, I decided to go out with a bunch of small, battery-powered Speedlites, and see what I was able to get out of them.
To make things easier on yourself, one way is to shoot with the sun behind your subject (or otherwise in the shadow). This way, because the subject is completely backlit, the side you're shooting is almost completely dark, so it is easier to control the light.
The first thing to ascertain is your exposure for the background—regardless of what happens to your subject at this point. This is what the scene looked like when exposing for the background:
Exposing for the background—no flash yet. (1/160 @ ƒ/5.6, ISO 100)
Naturally, to use flash, your shutter speed must not go faster than your camera's X-sync (somewhere typically in the order of 1/200-1/250)(1), so if the ambient light is too strong, you have to resort to very small apertures to compensate. This creates a very deep depth-of-field, which is not necessarily what you have in mind, if you wish to separate your subject from the background. In this case, I used a 3-stop (or was it a 4-stop? I can't quite remember) ND filter on the lens to cut lots of light. This allowed me to use a reasonably wide aperture on my 16-35mm zoom(2).
An early attempt. The sun is in the frame, which is an interesting effect, despite the flare/ghosts it creates.
For the main light, I used a simple bounce umbrella. I quickly realized that a single Speedlite 580EX II wasn't going to be enough, so I threw a second one at full power in there. What's more, the umbrella was relatively close to the subject, otherwise there simply wasn't enough juice.
Because the scene is back lit, the ground is rather dark. I needed separation between my subject and the dark background, or else she would have been lost. To do so, I added smaller Speedlites 430EX II from behind, on each side, as rim lights (with a gobo on each, to prevent the lights from hitting the lens directly, causing a lot of flare). Because they are bare lights, they don't need nearly as much power.
Rough diagram of the light setup. (Sun not to scale.)
I asked her to try whipping her hair, to see what interesting result we could get. I ended up keeping that image, as the light from the sun caught in her hair:
I would have preferred softer lights in the back (strip soft boxes, for example), to avoid the sharp shadows the bare lights caused (look around her neck), but there is no way these small lights would have had enough power. Another thing that became quickly apparent is that Canon's proprietary line-of-sight remote flash triggering doesn't work well in bright daylight—the flashes just wouldn't trigger even at such an arguably short distance. Radio triggers work well, but they force you to go manual.
To light at roughly the same angle as the sun, you need a good deal of artificially created photons. In this case, I used two Speedlite 580EX II at full power in a shoot-through umbrella, just outside the frame at camera right. You can see the "double" shadow caused by the sun (look at her nose and glasses):
Overpowering the sun. (1/200 @ ƒ/6.3, ISO 200)
Here, again, I employed some ND filtering to reduce the aperture I would have had to use to keep my shutter at a usable speed. I avoided the triggering problem I faced during the previous shoot, this time using a long E-TTL cable that ran from the camera directly to one of the two flashes.
A careful observer will note that the buildings in the back do not exhibit converging lines. This is because I actually shot this with a TS lens that had a small amount of rise (shift up) to correct for it. (Why make things simple when you can make them complicated?)
Is it possible to use small flash guns in bright daylight? Yes, on the condition that you place the source very close to your subject. It can therefore work decently for a tight shot of a single person, but for a loose framing or a group of people, it would become difficult (unless you are using a considerable collection of flashes). Moreover, because you are using those flashes at (or near) full power, you are pushing them quite a bit. Bring spare batteries, and watch out for the sun hitting them directly and getting them even warmer.
While the light weight of small flashes is an advantage for transport, it actually plays against you in the field—it doesn't take a lot of wind to topple an umbrella on a stand. (During the second shoot, I was holding down the light stand with my right foot, while applying a lens adjustment and focusing manually, handheld... Do as I say, not as I do.)
(1) Yes, you can use FP/High-Speed Sync, but this dramatically reduces the output of your flash(es). You end up needing a small army of Speedlites to get enough power, which is not necessarily an option.
(2) Shooting through lots of neutral density, in particular when the sun is shining right into the lens, strains your camera's autofocus system. I sometimes need to block the sun with my left hand, focus, and then shoot, otherwise the camera just won't focus. This is less than ideal, especially when dealing with a very narrow depth-of-field...