Teaching a class on location lighting, we mimicked Renaissance figure-in-the-landscape painting a bit by making sure that everything from foreground to background was in focus (at ƒ22!) and balancing the exposure on the figure to the background by firing a portable flash through a 42" flex-fill diffuser held as close as possible to the model. This version used a shorter shutter speed to darken the background without affecting the figure, which heightens the drama but also draws more attention to the lighting.
Why use flash outdoors when there is plenty of sunlight? Well, there's a whole book of reasons, but there's a very simple reason when you're photographing someone outdoors: it's impossible to hold an ideal exposure on someone's face and the sky behind them. Simply put, the sky is much brighter than you realize. You know the result: you end up with silhouettes or dark faces, or you have to over-expose for faces and settle for blown-out skies. So a basic use of strobe is adding fill flash on your foreground subject to balance to the brightness of the sky, as in this example.
For those of you new to mixing flash with ambient, continuous light, the hard thing to wrap your mind around is that your shutter speed has no effect on the exposure of the flash, because the duration of the flash is so short. Battery-powered strobes, which don't have to be so bright, are much shorter than most studio strobes; the Vivitar 285HV used in this shot can be as short as 1/30,000 of a second. If you were shooting in a dark room, that means you could leave the shutter open for 1/30,000 of a second or all day, and you'd still get the same exposure, because the lights are only on for that 1/30,000 of a second!
Ambient light from the sun or a light bulb, on the other hand, is
continuous, so the longer you leave the shutter open, the more light collects on your subject, and thus the brighter the exposure. Leave your shutter open too long and so much ambient light builds up that it obliterates the effect of your flash.
There is, however, a limitation on how
short your shutter speed can be,
not in terms of affecting exposure, but because the only way a camera
can manage extremely short shutter speeds is to expose one little piece of
the sensor or film at a time. This is accomplished by using two
"curtains" which admit a
sliver of light as the slit travels quickly
across your sensor or film. Unfortunately, that means that with very
short bursts of strobe light, only the little slit that was open to the
light will get any exposure at the time the flash fires!
Below is a demonstration: exactly the same ISO and aperture, but with progressively shorter shutter speeds. You'll notice the background (which the flash obviously doesn't reach) getting darker, while the strobe light on Paulina is unaffected. (The last one is darker all around because the assistant didn't hold the flash at the same distance from the model. You may also notice the addition of a 2nd strobe edge lighting her in two of the shots.)
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| ISO320 f22 1/60th |
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| ISO320 f22 1/80th |
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| ISO320 f22 1/100th |
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| ISO320 f22 1/200th |
On most modern D-SLR cameras with what's called a focal-plane shutter, the shortest shutter speed at which you can sync a flash is around 1/200th of a second, although you may find it to be a little finicky, more like 1/160th. However, if you use a camera that uses lenses that have a
leaf shutter in the lens, you are able to sync with flash at
any speed. Unfortunately, the shortest shutter speed that most leaf shutters provide is 1/500th of a second. With flash and ambient light, that means you can only darken the exposure of the background up to a certain point.
The other thing that will be new to people who haven't used anything except on-camera flash is taking your flash off camera to get away from that deer-in-the-headlights look. You can
trigger it very inexpensively by connecting a PC sync cable - IF both your camera and your flash have a socket, and if you don't mind tripping over the cable.
Otherwise,
wireless is the way to go. The inexpensive option I recommend is the
Interfit Strobie iSync 4 transmitter/receiver. The transmitter and receiver are $
60 total - peanuts. They're tiny, and the range is rated up to 100 feet, which will suit most applications. I like the hot shoe mount on the receiver, and the 1/4" light stand socket in the bottom, which bypasses the weak link - the fragile PC cable.
The preferred tool of pros is the
Pocket Wizard system, which also has four channels but a range of about
1600 feet! In my experience, more reliable than the Strobies, but far more bulky and expensive at about $
350 for a transmitter and receiver. (You need a minimum of two, one on the camera and one attached to each flash.) Your flash will also need a PC socket in order to connect to the Pocket Wizard via a short cable (included).
If your flash doesn't have a PC socket, you'll need to get an inexpensive hot shoe to PC sync adaptor.
The other basic lighting tool you'll want to invest in is a collapsible flex-fill, preferably one that has soft white and hard silver reflectors and a diffuser.
So to summarize: when you're mixing ambient light and flash, as long as your shutter isn't so long that the ambient light becomes brighter on your subject than your flash, you can control the brightness of the part of the scene that the flash
doesn't reach simply by changing your shutter speed. Aperture and ISO will still affect the exposure of everything.
