First and second-curtain sync

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It's worth reading the previous two posts before tackling this one. Except for when using FP mode on your Speedlite, the shutter is fully open when the flash fires. The shutter is fully open when the first curtain has come to rest, and before the second curtain has started moving - the time that the shutter is open is at least 10x longer than the flash duration. When does the flash fire? By default, at the first possible opportunity - when the first curtain has come to rest and the shutter has just opened. This is termed first-curtain sync. More sophisticated flash units also give you the opportunity to fire the flash just before the second curtain starts moving and the shutter starts to close - second-curtain sync. This is accessible on the 580EX II Speedlite by the button just below the red box shown in the previous post.

To see the impact of curtain sync, check out the images above. The camera was in aperture priority mode (the flash behaves differently depending upon which mode you're in - a subject for a future post) and to expose for the ambient light conditions a shutter speed of 1/2 sec was required - using flash with long exposure times to record ambient light is known as 'dragging the shutter'. In the first example I set the toy rolling (right to left) and used a remote shutter release to trigger the camera, mounted on a tripod, without firing the flash - the toy travelled for the entire duration of the shot and hence is completely blurred. In the second example I switched on the flash. It fired as soon as the first curtain had come to rest, freezing the motion of the toy, and then the shutter stayed open (to record the ambient light) for the remainder of the 1/2 sec exposure time - thus a sharp image of the toy was recorded (when the flash fired at the beginning of the exposure) followed by a blur - the result of which makes the toy appear as if it is travelling backwards. In the third example I set the Speedlite to second-curtain sync. Now a blurred image of the toy was recorded until the flash fired just at the last moment (before the second curtain started to move and close the shutter) to record a sharp image - in this case, the toy appears to be travelling forwards and gives a visually more appealing sense of movement.

Capiche? Please post a comment if further explanation is required.

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