Function determines what shutter speeds the camera can select when you use flash in the Av (aperture-priority autoexposure) mode. At the default setting, which is Auto, the shutter may be set to any speed between 30 seconds and 1/200 second. But you can also limit the camera to a shutter speed of 1/200 second through Custom Function 3. Doing so eliminates the possibility that a slow shutter speed will result in a blurry image due to camera shake, but it can also result in dark backgrounds. See the earlier section "Understanding your camera's approach to flash" for details about using flash in the various exposure modes.
1 Custom Function 8, E-TTL II: This option enables you to switch from the default flash metering approach, called Evaluative. In this mode, the camera operates as described in the earlier section, "Understanding your camera's approach to flash." That is, it exposes the background using ambient light when possible and then sets the flash power to serve as fill light on the subject.
If you set this Custom Function instead to the Average option, the flash is used as the primary light source, meaning that the flash power is set to expose the entire scene without relying on ambient light. Typically, this results in a more powerful (and possibly harsh) flash lighting and dark backgrounds.
1 Custom Function 9, Shutter Curtain Sync: By default, the flash fires at the beginning of the exposure. This flash timing, known as first-curtain sync, is the best choice for most subjects. However, if you use a very slow shutter speed and you're photographing a moving object, first-curtain sync causes the blur that results from the motion to appear in front of the object, which doesn't make much visual sense.
To solve this problem, Custom Function 9 enables you to change the flash sync timing to second-curtain sync, also known as rear-curtain sync.
In that flash mode, the motion trails will appear behind the moving object. The flash actually fires twice in this mode: once when you press the shutter button and again at the end of the exposure.
As you can probably discern from these descriptions, these Custom Functions are designed for photographers who are schooled in flash photography and want to mess around with advanced flash options. If you fall into that category, you can access the options by displaying Setup Menu 2, highlighting Custom Functions, and then pressing the Set button. After the Custom Function screen appears, use the right or left cross key to select the function you want to change. For example, in the left image in Figure 5-28, I selected Custom Function 8; in the right image, Custom Function 9.
Shutter curtain sync.
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Figure 5-28: Both of these Custom Functions are designed for advanced users.
Press Set to activate the scrolling list of options in the center of the screen, as shown in the figures. Press the up or down cross key to select your choice and then press Set again to lock in that option.
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