I thought it had to do with timing of the flash and the shutter. Slow sync and the shutter are timed differently to expose more background, but movement can cause trails. Regular sync prevents this problem, but the background is not exposed as well. Maybe i'm out to lunch, but i've tried slow sync and reg sync with stationary subjects, and there is quite a difference.
Brad
Slow sync is nothing more than a mode that lets you shoot with a shutter speed slower than normal. It doesn't have anything to do with the timing of the shutter.
Nikon has decided that the general public isn't quite smart enough to understand how their camera gear works; I suppose that's true to some degree. So when Nikon built the current crop of consumer grade dSLRs they limited the minimum shutter speed to 1/60sec. That is to say, if you indicate to the camera you intend to use flash (by raising the pop up for example) the camera will try to prevent you from using a shutter speed below 1/60 sec. The idea is to prevent too much image blur, especially if there is a good deal of ambient light. Of course, the camera will also prevent you from exceeding the maximum sync speed too. That's usually 1/200 or 1/25 sec depending on the camera model.
There are two ways to change this minimum shutter speed limitation. One is to go into the menu settings and select a different value. The other is to select "Slow Sync", which removes the limitation altogether and lets you use as slow a shutter speed as you desire.
Neither of these change the shutter and flash timing. Under normal circumstances the flash fires just after the shutter is fully open. In almost every situation, the flash duration MUCH shorter than the length of time the shutter is open.
In other words, the flash does it's job right at the beginning of the exposure. Any further exposure for ambient occurs during the rest of the exposure, which of course is controlled by the shutter speed. The process ends when the second curtain closes.
What you are observing with Slow Sync are simply photographs made at slower shutter speeds. If they are properly exposed, you will notice that ambient light plays a bigger role, i.e. the background gets MORE exposure. Naturally, if there is significant ambient light, moving subjects will be blurry because of the slow shutter speed.
On the opposite side of the coin, with a properly exposed image using higher (shorter) shutter speeds, the flash plays a larger relative role so generally speaking the background gets LESS exposure. Most likely you will see less subject blur due to movement.
Both the ratio between subject and background lighting and the differences in how motion blur is captured can be good or bad depending on the effect you're after.
None of this is rocket surgery. Many of us consider Nikon's limitation on minimum shutter speed with flash and the inclusion of Slow Sync mode annoying for the experienced photographer and confusing for the beginner. I'd rather not have it at all.
In any case, the timing of the flash and shutter remains the same.
Now Rear (sometimes known as second) Curtain Sync is something completely different from Slow Sync and a subject for another thread.
I hope this helps rather than having the opposite effect.:biggrin:
