Slow Rear Flash?... How it works?

[Galileus]

I always wondered about this and I'd like to know in what scenarios/cases to use it and how it works...

Thanks for your time.

José.

[Olivier H]

The flash output duration lies between 1/2000 and 1/10000 of a second. So the shutter speed does not count if slower than this (usually 1/250 to 1/500 at max), only the aperture.
Basically, when you are in Auto/Program mode, the flash will be high speed synchro, above 1/60s to avoid camera shake as much as possible.
But, if you want to include "ambient" light, that is available continuous light input, you might want to have a longer shutter speed (1/20s in bulb lit interior). This is what the SLOW synchro mode is for. To tell the camera : hey, no matter if the flash is going to fire, I want you to meter for ambient light.
Of course, with auto/TTL flashes, the onboard computer will also adjust the flash output so that when it adds to the ambient, everything is not blown out.

The SLOW mode is what is called front-curtain sync. That means the shutter opens, the flash immediately fires, then the rest of the shutter opening time is used to gather light from the ambient.
If things or people are moving, they will be freezed by the flash at the beginning of their movement, and have a motion blur trail AFTER the moment they are frozen.

To have a motion blur that describes what happened BEFORE the freeze, you use the REAR curtain sync, that is that the flash fires just before the shutter closes. This is also a slow sync mode.

Hope this is clear enough.

There is a post on DPS but I can't find it.

[Olivier H]

Here it is :
http://digital-photography-school.com/blog/slow-sync-flash/