Allison Earnest explains how to take control of the balance between background and subject exposure on a location shoot in this excerpt from her Amherst Media book The Digital Photographer's Guide to Light Modifiers.
This excerpt from The Digital Photographer's Guide to Light Modifiers is provided courtesy of Amherst Media. To purchase the book and learn more about the publisher, visit the Amherst Media website.
Many photographers do not fully understand their camera’s exposure capabilities when incorporating a hot-shoe flash unit. As a result, most use their flash on-camera and let the camera do all the work by shooting in program mode and TTL. This is fine for the hobbyist, but it’s unacceptable if you’re entering photography as a profession.
When balancing flash and ambient light, your shutter speed controls the ambient-light exposure, while your aperture controls the flash exposure on your subject. For example, imagine you are photographing a subject where the ambient light meters 1/100 second and your subject exposure, using a flash, measures f/8. If the composition stayed the same and you reduced your shutter speed to 1/30 second, the only change in the photo would be a lighter background. The exposure on your subject would be unchanged. Conversely, if you kept your shutter speed at 1/100 second and opened up your aperture to f/5.6, your subject would record as one stop brighter but your background would remain the same or show very little change.
There are several exposure techniques that you can choose when using hot-shoe flash. Some photographers prefer to use aperture priority mode (A on Nikon; Av on Canon), which allows you to control the aperture (the subject exposure) while letting the camera choose the proper shutter speed (the ambient-light exposure).
Personally, when I’m not shooting in the manual mode (where I choose both the shutter speed and aperture myself), I always use the shutter priority mode (S on Nikon; Tv on Canon). There is no right or wrong technique; I’ll explain my preferred method.
Whether inside or outside, the available light typically doesn’t change or changes very little. Taking a meter reading with a handheld meter allows me to measure the available light level and set my shutter speed accordingly. With today’s sophisticated flashes and their incredible TTL (through-the-lens) metering capabilities, I typically control the exposure of my flash by increasing or decreasing the power of the flashes via the Nikon’s commander function. By using the shutter priority mode, I take one variable out of the equation. Because I have control of the shutter speed, I can control the available light throughout my image by increasing or decreasing my shutter speed—thus changing the relative density of my background to suit my taste. There are many different ways to arrive at the same conclusion, but this is how I typically expose my images when not shooting in manual mode. You may want to experiment on your own and find which technique works best for you.
Whatever method you choose, I would not recommend shooting in program mode. When you do, you have no control over your photographs and you will be at the mercy of the camera’s final exposure. Let’s look at some examples.
Photograph 5-19, featuring model/actress Irena Murphy, was shot on program mode (with no flash) when the sun was low in the sky. The program mode worked; this is a “correct” but average exposure of the scene. If I had another photographer shooting next to me in program mode, we would both have taken the same snapshot (assuming we were using the same focal length).
Photograph 5-19. Subject: Irena Murphy. Camera: Nikon D300. Settings: Program mode, 1/160 second, f/6.3, ISO 400.
Let’s say I zoomed into the image, as in photograph 5-20, still using the program mode but with the addition of TTL flash. Now, the meter in the camera sees a lot of black (the background is a submarine) and a backlit subject. As a result, the camera overexposes the subject to compensate for the dark background, dark clothing, and shadow in the image. The camera produces an overall well-exposed image—although it over-flashed the subject and the image lacks dimension. This is an example of what not to do.
Photograph 5-20. Subject: Irena Murphy. Camera: Nikon D300. Settings: Program mode, 1/250 second, f/10, ISO 400.
Taking better control of this scene isn’t difficult. I placed my camera on manual and took a meter reading for the ambient light (photograph 5-21). This gave me a shutter speed of 1/60 second. I desired a lighter background, so I adjusted my shutter speed to 1/40 second to add more light on the background. My flash exposure was f/18 with my SB800 bounced off a LumiQuest modifier. The power on my flash was reduced by one stop. The result is a beautiful portrait that was created, not "taken."
Photograph 5-21. Subject: Irena Murphy. Camera: Nikon D300. Settings: Manual mode, 1/40 second, f/18, ISO 400.
When photographing interior portraits, my preferred method of shooting is shutter priority mode. It's also useful when I am rushed—which was the case when creating photograph 5-22. Irena and I were on a Russian submarine tour in California and had very little time to create this image. Using the shutter priority mode enabled me to take one calculation (the aperture) out of my process. For this interior image of Irena, an ISO of 2000 was required in order to handhold at my desired shutter speed (1/20 second). I handheld the image because I did not have a tripod and the submarine was a very tight working space. At f/5, the power on my SB800 flash (the main light) was reduced by one stop. A second SB800 flash, powered two stops below the ambient light and fitted with a warming gel, was placed approximately 90 degrees to camera left. Can you see the warm fill on Irena's right cheek? Had this image been shot in the program mode, the right portion of the scene would have been underexposed and rendered as a black hole, a non-dimensional snapshot.
Photograph 5-22. Subject: Irena Murphy. Camera: Nikon D300. Settings: Shutter priority mode, 1/20 second, f/5, ISO 2000.
I should note that the model's Russian hat was borrowed from the gift shop with advance permission. Without it as a prop, this image would not have been as dramatic or convincing. Don't be afraid to ask—the worst thing that can happen is they say no.