Brett wrote in post #8516001
If Rear Curtain Sync was not used, you'd have trails off the subject people after the flash fires.
...they're still relatively frozen by the flash firing right before the rear curtain closes, with little or no trails.
This is exactly what I meant. Many seem to believe that there's some difference in lighting in situations like this one, depending upon whether the flash fired at the beginning of the exposure or at the end. But there is not, something which should be evident if you look around, while you hold a flash unit in your hand and test fire it (using the PILOT button). Since the other exposure the camera does doesn't affect the environment at all, you can just as well simulate this with the flash only. You'll quickly notice that things that aren't glowing by themselves leave just about the same kind of trails before as after the flash fired. There's no difference to the lamps in the ceiling either, just because you did or did not fire the flash.
The exposure on the sensor (or film, if you would use that) doesn't know if light hit it "now" or "then", so it doesn't matter if the flash came early or late. It will overpower the ambient anyway, so people lit by the flash will appear frozen, no matter when the flash fired. The trails they leave will be just as obvious or faint, regardless of whether they were recorded before or after the flash.
However, if you are taking photos of a moving object, without panning, and this object has lights of its own (like a car), then you'll see the difference. The lights will be clearly visible, leaving streaks of bright light in the image. If you then fire the flash at the beginning of the exposure, it will look like the car runs backwards, after the photo was taken. 2nd curtain sync will take care of that.
But with people moving randomly in a night club there will be no difference for this reason. The only reason for using 2nd sync there, and that could indeed be a good reason, is that people will not be aware of you taking a picture until it's already done. If they see the flash at the beginning of the one second exposure, they may do something else in the image than they otherwise would do, if they don't know you pushed the trigger. At least if you don't use AF and the associated AF assist beam from your flash, taking a picture in a (usually) noisy night club will go unnoticed by everyone not directly seeing your camera, until the flash actually fires.
So there could very well be a reason for using 2nd curtain sync in a night club, but as a photographer, you should know what that reason is, so you know what you are doing.
