Photographers who push their boundaries particularly in the realm of flash are always looking for ways to enhance their work. I have recently been approached a fair bit about the value of second or rear curtain sync and the difficulty of getting it working with Canon cameras.
First, what's the big deal? If you make all your flash exposures at shutter speeds above 1/60 of a second or so, there's no big deal at all. But let's suppose you are mixing flash and ambient light in a long exposure of cars on a street at night, or dancers at a wedding reception. In traditional first curtain flash synchronization the flash goes off as soon as the shutter is opened. You get a flash exposure and then the shutter stays open for the set shutter speed and closes when the time is up. In this time, the cars or people may have moved to creative effect. But because the flash went off at the beginning, the first part of the movement is frozen, and then the subject movement shows from that point. In the car example, you get a nice sharp car with light streaks moving forward of it. In the dancer example, you get a sharp couple, with the blur of their movements overlaid. Now if you were to use rear / second curtain sync, you would get the motion first and then the sharp freeze of action, so the light trails would be coming out the rear of the car, and the dancing couple would show the blur of motion right up to the flash and freeze point. These are just two examples where second curtain sync is a real boon.
Let me step back a bit in time. In older Canon DSLRs, rear or second curtain sync was available on some models. I still shoot with a 1D Mark IV and it's very elegant and simple. In one of the custom menus, you simply choose second curtain sync. When I was a student with Joe McNally, he noted in a class that 80% of his flash images are using second curtain sync, it's a conscious choice.
The Nikon D4s I am working with right now for the evaluation reminds me of the 1D Mk IV. Rear/second curtain sync is simple a configuration choice. The way it should be.
And if you happen to use Canon Speedlites on some Canon cameras in the hotshoe, in your external flash control menu, you can still select a rear curtain sync. Which is great, except for the part about on camera flash typically looking like crap. We want off camera flash. But according to the geniuses at Canon the only way to do that is using the short and expensive OC-3 cable. When I pestered a Canon engineer, he agreed that second curtain sync with wireless was missed in Canon wireless flash systems be they infrared or even the new radio based 600 EX-RT system. His belief was that this could be changed in firmware, but 12 months later, not a shred of movement. And it's got nothing to do with the camera, it's the flash system itself. How do I know this? If I use a YongNuo YN-E3-RT radio transmitter instead of Canon's own ST-E3-RT transmitter, I can actually have rear curtain sync wirelessly so long as I am using the flashes in manual. In fact if I use YongNuo or Phottix TTL triggers and receivers I can still have second curtain sync in TTL mode. My Profoto B1 Airs with AirTTL transmitter do TTL second curtain sync. It's possible but Canon just doesn't get it.
Unlike the third party units that are firmware upgradeable via USB, Canon leaves this out of their own flash units so there is no way for them to add functionality that they forgot. As a forward thinking company, Canon has dropped the ball here big time. If I bought the YongNuo 600EX clone, I can get three for the price of the single 600EX-RT, it will be fully compatible and it will take upgrades via firmware, although YongNuo has yet to figure out that lots of photographers are Macintosh users and their firmware tools are Windows only.
If I went with the excellent Phottix Mitros+ radio flash system, they are not radio compatible with Canon, but they are firmware upgradeable.
But wait, there's more idiocy. Since the dawn of flash time there have been these things called optical slaves. Basically they see a bright flash, they trigger the flash that they are connected to. Every modern flash has this capability. Unless it comes from Canon. Oh, you'll find the words optical slave in the brochure and the manual, but this is Canonese. It means infrared controlled slave from another Canon infrared transmitter. It does not mean optical slave in the classic sense. I had borrowed both a Phottix Mitros and a Nikon SB-910 for the D4s eval. Both have true optical slave as well as infrared slave, so plainly the two functions can coexist.
I have written up the Canon radio system and done a video on it. It's really very well done. However, Canon has with intent debilitated their flash systems by leaving out capability for rear curtain sync in all modes and by leaving optical slave triggering out of their flashes, especially the top of the line 600 EX-RT. I could even sort of tolerate the premise of leaving these things out of their entry level products to "help prevent user confusion" as one marketing rep explained it, but to leave it out of your pro level gear is just asinine.
So where does that leave us today? Radio triggering is far more reliable and effective than infrared, so you want radio. TTL from all vendors will save you time and hassle and be there at least 85% of the time without fiddling, so you want TTL. You also want to get the darn flash off the camera so a radio capable TTL trigger for the hotshoe would be a good idea. And it has to support second curtain sync if the camera can do so. Finally, the flash unit has to have an optical trigger so you can make go blink without wires, special transmitters or menu voodoo.
So if you are a Canon user and you want all that stuff what do you do? For the money, the YongNuo solutions kick butt. If you want full radio, including zones / groups with different control settings for each zone like Canon does so well, but you also want proper slaving and that second curtain stuff, you should look at the Phottix Mitros+ system. Either option is less money than the Canon variants and more feature rich.