Second Curtain Sync
Second curtain sync is important when you want to capture images showing motion blur. Most other times you won’t need it.
In order to understand second curtain sync we need to look at the camera’s shutter mechanism. Most DSLRs will have a shutter that sits in front of the image sensor preventing light from hitting the sensor when you’re not taking a photo. The shutter is made up of 2 screens known as curtains. When you press the shutter to take a photo the first curtain moves up and out of the way to let light pass through on to the image sensor where it is captured to produce an image. It’s a bit like a curtain at a theatre going up to reveal the stage at the start of a show. When enough light has passed onto the sensor to give a correct exposure a second curtain then moves up to block the light (not like as in a theatre unless you imagine that during the performance there was a second curtain lying on the stage floor and when the performance finished this curtain was suddenly pulled up to hide the stage). Anyway after the second curtain is fully up this marks the end of the exposure. Both curtains are then dropped down together and reset to their original position at the bottom waiting for another photo to be taken.
First curtain up, sensor exposed to the light, second curtain up, sensor blocked from the light, both curtains down, ready for another shot.
Normally if we use flash the flash fires at the moment the first curtain goes up. If it was a long exposure, say 5 seconds the first curtain goes up, the flash fires and lights up the subject, we then have to wait 5 seconds to capture the ambient light, then the second curtain comes up, the exposure is complete and both curtain reset to their original position. The flash from the speedlite is almost instant (at quarter power it’s around 1/2000 sec and at 1/128 power it can be as quick as 1/10,000 sec) and is faster enough to freeze any motion. Since the flash fires as soon as the first curtain is open this is known as first curtain sync.
Second curtain sync is when the flash is fired just before the second curtain goes up to cover the sensor. i.e the shutter is pressed, the first curtain goes up, we wait for the ambient light exposure, the flash fires and then the second curtain goes up.