Flash synchronization is the coordination of the short burst of light from the flash with the longer overall exposure time.
To understand it, it is important the burst of the flash is about 1/1000 s or les and a lot shorter than the total exposure time during which light falls onto the image sensor.
The different options of flash synchronization are:
The Standard: “1st Curtain”
The flash is fired when the exposure starts as indicated in the chart on the right.
This synchronization is also called „1st curtain“ because the usual shutters open sideways like a curtain.
„2nd Curtain“ or „Rear“ Synchronization
The flash is fired at the end of the exposure time, just before the „curtain“ starts to close.
The difference to the first case is that blurred motion from the longer exposure time will appear behind your subject which looks more natural.
On the left a sample picture with the common synchronization on 1st curtain on top, below one with synchronization on the 2nd curtain.
The little toy car with remote control made the same movement forward for both pictures.
Reduce red eyes
The flash is used to blink before the image is taken so that the pupils of people on your image contract and the risk of read eyes from the flash gets smaller.
The actual exposure gets delayed which is a significant disadvantage of this function.
This function can be combined with both of the first two options for synchronization.
The exposure time is adjusted to the dark background and can get as long as needed, well beyond the limitation of usually 1/60 s.
You will get a brighter background but also long exposure times with the risk of blurs from camera shaking.
The typical situation to use this are night shots with long exposure time for a nice background and the flash lightens a person in the foreground.
Here an example of Freddy during an evening walk.
About half an hour after sunset the sky needed half a second of exposure time to appear as dark blue while the usual maximum exposure time of 1/60 s leaves it black.
This type of synchronization can be combined with all three others before. For practical reasons it is good to use it with the synchronization on the 2nd curtain because people will think that the image is taken when the flash has fired and then start to move. But this disturbs your image if the camera continues to expose for another second or so.