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Controlling Ambient In Flash Exposures

As a Speedliter, there will be times when you want to accentuate the role of ambient light in your photographs, and there will be times when you want to minimize the role of ambient light in your photographs.

If you can come to understand the following, you will have mastered one of the hardest concepts about flash photography:

  • Use the shutter to control the ambient exposure.
  • Use the aperture to control the flash exposure.

If you overthink it, you'll fall into the trap that has confused legions of photographers. So, if you can't get your head around it, then just accept these truths and get on with shooting. The key is to use a faster shutter speed if you want to dim the ambient and a slower shutter speed when you want to collect more ambient (typically in dim light).

Why Shutter Does Not Control Flash Exposure

I know it's hard to believe, but it's true: as long as you are shooting at or slower than the sync speed for your camera, then shutter speed has no control over the amount of Speedlite flash hitting your digital sensor. If you shoot faster than your camera's sync speed, you are not reducing the amount of Speedlite flash hitting the sensor; rather, you are only reducing the portion of the sensor that is exposed.

Here's why the shutter has no effect on the amount of flash getting through: a Speedlite's longest duration of flash is produced at full power—which is reported by Canon to be 1by800_inches.jpg. This is much faster than your camera's sync speed (typically 1by250_inches.jpg).

Take a look at Figure 4.4. The width of the black box represents 1by250_inches.jpg—the sync speed of many Canon DSLRs. The green line is the flash duration of a 580EX II at 1/2-power The red line is the flash duration at 1by128.jpg-power. The first thing you should notice is that the higher the power setting, the longer the flash burst. The second thing you should notice is that the each is just a sliver of the width of the frame. Full power, although longer than ½-power, is still much narrower than the camera's sync speed.

Figure 4.4

Figure 4.4 Two flash durations of a 580EX II as measured on an oscilloscope. The green line is 1/2-power. The red line is 1/128 power. The width of the frame is 1/250 —the sync speed for many DLSRs.

So it does not matter if you're shooting at 1by250_inches.jpg or 1/2;", the flash from your Speedlite is flying through the lens for a mere fraction of the entire exposure. (If you want to shoot faster than 1by250_inches.jpg, you'll have to switch your Speedlite over to high-speed sync, which changes the game completely. For now, we're keeping it basic.)

Why Aperture Controls Flash Exposure

Aperture, on the other hand, limits all the light coming through a lens. If the shutter speed is 1" or 1by8000_inches.jpg, it does not matter. All the light, ambient and flash, must pass through the aperture.

So aperture has a direct effect on the amount of flash hitting the sensor. If you are at full power and still need more flash, open the aperture by a stop or two. Then, to keep the ambient light the same, speed up the shutter speed by the same number of stops. Just remember, you can't exceed the sync speed for your camera without using high-speed sync.

Why Shutter Speed Controls Ambient Exposure

You already know that if you switch your shutter speed from 1by250_inches.jpg to 1by500_inches.jpg, you've just reduced the amount of sunlight getting to the sensor by half—you have reduced the amount of ambient exposure by half.

It does not matter if the ambient light is sunlight, room lights, or firelight—the shutter controls how much of it gets through the camera.

Now don't get hung up on the fact that the aperture also affects the ambient exposure. The thinking here is that if the aperture is set to control the flash exposure, you can adjust the shutter to control the ambient exposure.

As Speedliters, we will typically want to change the amount of ambient light in one of two ways:

  • In brightly lit scenes, we will want to reduce the amount of ambient so as to make the subject (lit by flash) more prominent.
  • In dimly lit scenes, we will want to collect more of the ambient light from the background so that the subject (lit by flash) is not standing against a black background.
Figure 4.5

Figure 4.5 The baseline shot at f/9, 1/200", ISO 100.

Figure 4.6

Figure 4.6 f/2.8, 1/1600"—2/3 stops darker than 4.5.

Figure 4.7

Figure 4.7 f/2.8, 1/6400" —1 2/3 stops darker than 4.5.

Figure 4.8

Figure 4.8 f/9, 1/1600 —3 stops darker than 4.5.

Let's take a detailed look at each of these scenarios. In the first series, you can see that changing the shutter speed in this outdoor shot has helped saturate the sky and increase the separation between Tony (son #3) and the background. Note in each of these photos that the power of the Speedlite, its distance to the subject, and the aperture have stayed the same. The only thing changing is the shutter speed.

Now we'll look at a series of shots made 45 minutes after sunset. Here, I've locked the camera down on a tripod to facilitate longer exposures. At 1by15_inches.jpg it's definitely a night shot.

Figure 4.9

Figure 4.9 1/15" —definitely a night shot.

Figure 4.10

Figure 4.10 1/2"—3 stops more ambient than 4.9.

Figure 4.11

Figure 4.11 2"—5 stops more ambient than 4.9.

Figure 4.12

Figure 4.12 8"—7 stops more ambient than 4.9

At 8" the scene looks almost like daylight. By using slower shutter speeds—sometimes described as "dragging the shutter"—I'm able to collect more of the ambient light from the background.

ISO Affects Flash And Ambient Equally

The ISO setting affects the "volume" of the signal coming from the digital sensor. It does not distinguish between flash and ambient light. If you double the ISO, then you've increased the exposure to ambient and flash by one stop.

Remember, in order to keep digital noise to a minimum, it's best to shoot at lower ISOs than higher ISOs. So think of ISO as a way to get your exposure into a range that you want:

  • If you want to shoot at a smaller aperture to increase depth of field and have your Speedlite at full power, you can increase the ISO.
  • If you need to get the shutter speed to a point that you can handhold without camera shake, you can increase the ISO.

Strategies For Aperture And Shutter With Flash

Now if you are thinking, "Why not just change the power setting on my Speedlite and not worry about the aperture?" it's because you will encounter situations where changing the aperture is preferable to changing the power setting.

Let's say that your Speedlite is at full power and you want to increase the effect of the flash on your subject while keeping the ambient at its current level. If you open up the aperture, you'll allow more of the flash to get through the lens. Then you can change the shutter speed by an offsetting amount (i.e., make it faster) to keep the ambient exposure the same.

Likewise, if your Speedlite is at minimum power and you still want less flash on your subject (a common situation in macro photography), you can close down the aperture to reduce the amount of flash getting through the lens. Again, you'll have to change the shutter speed by an equivalent amount (this time by going to a slower shutter speed) to keep the ambient exposure the same.

Another situation where it's easier to change the aperture than the power setting is when using Speedlites off-camera in manual mode. For example, you've set up an off-camera Speedlite for the new couple's walk down the aisle. As they enter the zone you're lighting with your Speedlite, you realize that you forgot to change the power setting on the flash. With no time to run over and change the power level, what can you do? That's right: quickly change the aperture to get the correct amount of flash, and change the shutter by an offsetting amount for the ambient.

Figure 4.13

Figure 4.13 When it was time to make a quick portrait of Tom Harris, owner of Harris Stage Lines, at sunset, the sun had dropped behind the barn, putting Tom in shade. This is the scene with only ambient light.

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