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4. Slow-Sync
Slow-sync flash just means combining flash with a slow shutter speed, generally to provide detail in a dark background, such as a night skyline, behind the flash-lit main subject. Some cameras do slow-sync automatically in dim light, and with some you have to set slow-sync mode.

(a, above) Normal front-curtain sync fires the flash at the start of a long exposure, then records ambient-light “speed streaks” as the subject moves across the frame. The result is an image with streaks in front of the subject (a). Rear-curtain sync fires the flash at the end of the exposure, after the ambient-light streaks have been recorded. The result is a photo with streaks behind the subject (b, below).

If you use a slow shutter speed to shoot a flash picture of a subject that is moving, the brief duration of the flash will sharply “freeze” the subject, while the long ambient-light exposure time will record a blurred “ghost image” in the same shot—a cool special effect.



With most cameras, the flash fires at the start of the exposure. This is known as “front-curtain” or “first-curtain” sync. If you make a long exposure of a subject that is moving across the frame, the flash will fire at the start of the exposure, “freezing” a sharp image of the subject; then the ambient light will register a ghost image of the subject as it moves across the frame. The result is a photo in which the ghost-image “speed streaks” appear in front of the subject—an unnatural effect. Some cameras also offer “rear-curtain” or “second-curtain” sync, in which the flash fires at the end of the long exposure rather than at the start. With rear sync, the ghost-image speed streaks will be registered by the ambient light as the subject moves across the frame, then the flash will fire to sharply “freeze” an image of the subject at the end of the exposure. The result is a photo in which the ghost-image speed streaks appear trailing the subject—a more-natural effect.

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