Slow Synchronization

When shooting with flashes, it is very likely that the flash light over powers the ambient, and, as a result, the image only records the flash illumination. In many cases, this is not what a photographer wants. For example, if we photograph a scene illuminated by incandescent lights with flash, it is likely that the flash will over power the incandescent light and make the warm tone to disappear. Or, if we photograph a scene with dim background with flash, it is likely that the foreground and the main subjects will be illuminated properly by the flash, and the background at a farther distance may still remain dark because flash light may not reach far enough. These results certainly do not look real. To overcome this problem, we can use flash and a slower shutter speed. Here, "slower" means slower than the flash synchronization shutter speed. But, why does this work? The answer is very simple. When the shutter opens fully and then flash fires, if the shutter closes immediately, the image only records the flash illumination. If the shutter opens a little longer, the image will also record some illumination by the ambient light. This added ambient may cause the portion that receives flash illumination to over-expose a little, but allows the severely under-exposed portion that does not receive flash illumination to expose to ambient. Therefore, the result will include some details that cannot be seen when the same scene is taken with flash only. This is slow synchronization, the use of a slower shutter speed to allow both the image to record flash and ambient illumination.