最初由 hongchen 发表 Av模式下闪光灯的工作方式
Av mode lets you set the depth of field by specifying the lens aperture. The camera then chooses a shutter speed ranging from 30 seconds to the camera’s X-sync speed, in order to expose the background correctly. If that means the shutter speed is some really low value so that you need to use a tripod to avoid camera-shake blur, so be it. In dark conditions, therefore, Av mode works in slow sync mode.
Av模式让你通过选择光圈来确定景深。相机选择快门速度从30秒到相机的同步速度，使背景产生真确的暴光。这意味着快门速度很慢，你不得不使用三脚架以避免手振。所以，在光线不足的条件下，Av模式工作在低同步模式下。
Flash duration (flash output) is determined by the flash metering system. Like Tv mode the camera always works in fill flash mode when in Av mode.
闪光时间（闪光输出）是由测光系统决定的。和Tv模式一样，在Av模式下，相机总是处在补光模式下。
There is one exception to this. A number of EOS cameras have a custom function you can set to ensure that the shutter speed in Av mode when using flash is locked to the X-sync speed. The EOS 10/10s and Elan II/EOS 50, for example, have such a custom function, which lets your camera behave more like P mode when in Av mode. However this custom function will only lock the camera to X-sync in Av mode and will not choose a shutter speed from 1/60 sec to X-sync, the way P mode does.
对此有一个例外，一些EOS相机在Av模式下使用闪光灯可以手动设定快门的速度为同步速度（注：400D有这个功能）。例如OS 10/10s和Elan II/EOS 50就有这个功能。这时你的相机更象是在P模式。但在这种情况下，只可以把快门速度设定为同步速度，而不能象P模式那样，速度在1/60秒到同步速度之间选一个合适的速度。
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