Conventional wisdom dictates that we keep our synchronization at Rear Curtain. The reason is simple. If we want to show movement "blur", we can do so by having the speedlight fire just as the rear curtain of the shutter closes. This is so the flash will light subject at the end of its journey and not at the beginning. Rear curtain synchronization leaves the blur "trail" behind the moving subject for a more believable image.
I had a case where this worked against me. I was photographing at Vault 164 in San Mateo during a fundraising dinner for the Police Athletic League. I wanted to photograph the two owners flanking the Chief of Police wiin front of a decorative shadow box that covered a back wall. The wall's insets were illuminated with accent lights, highlighting some of the gears and shafts recovered from the vault door (the restaurant used to be a bank).
First, I made a photograph of the background using available light. This would be my base exposure. the first shot was taken with a flash white balance, the second with a tungsten setting. Final setting was 1/10 of a second at F5.0. I was using a D300 and a Tokina 11-17mm lens at ISO 1600.
I then attached a Full CTO gel to an SB-800 in the remote mode with a Lumiquest Softbox III and put it on a monopod about 3 feet above the lens axis. I had an on-camera SB-800, directed at the ceiling, to give an overall lift to the exposure. I used a small piece of black cardboard on the flash head to prevent any direct light from spilling onto the subject's faces. Both the on-camera and off-camera flashes were set to TTL with no exposure compensation. I really didn't have time to mess with any settings, as it was getting crowded and noisy, so camera adjustments and verbal directions were getting difficult to execute. I suggested that the two owners point out some of the architectural details of the converted bank, and I will say the restaurant was beautifully done.
Now here's the one time when rear curtain synchronization was a definite drawback. The monitor preflash, the signal provided by the on-camera commander speedlight to instruct to remote units, happened while the mirror was in the down position. This allowed me to get a glimpse of how the final photo would look. But when the actual exposure was made, it came 1/10 of a second after my preview, so the shots I thought were well timed were noticeably "late". The room was dark enough so the subjects were only lit by the speedlight at an effective exposure of 1/1000 of a second, or shorter. The background is pretty sharp, considering the length of the exposure.
It took 5 minutes and 17 exposures to get one acceptable shot (#10 of the series). After each shot I "chimped" it, and found something wanting in those first 9. The last seven were attempts to cleanup the poses, but in end this was the best of the batch. I am glad I planned to get this shot early in the evening so I could leave the guests alone to enjoy the party.
Looking forward, the rear curtain synchronization setting could cause problems with longer exposures where the subject isn't moving. If the subject sees the preflash, they'll assume the photo has already been made, and that they can relax. This could result in some subject movment at the back end of the exposure. But if I instruct the subject to remain still after the initial flash has gone off, they know, hopefully, to hold still a while longer. Since I've got some night shots coming up, I shall test this theory to the utmost.
