What will happen when a Nikon SB-800 in SU-4 mode (dumb optical slave mode) is placed amount a group of Canon flashes that are setup for Canon Wireless SpeedLite Flash system triggered by the Canon ST-E2 Speedlite Transmitter?

That’s what one guy tried to do and the result was interesting: The SB-800 seemed to work ok but all Canon flashes (a 580EX and a 430EX) refused to fire! How could this be?

Well, I offered an answer that is plausible. Basically, the Canon Wireless SpeedLite System (WSS) works in a similar fashion as the Nikon Advanced Wireless Lighting (AWL) system. The ST-E2 is equivalent to Nikon’s SU-800. Both systems use series of infrared pulses to communicate with and trigger the remote flashes. Canon’s communication protocol is incompatible with Nikon’s. That’s not the problem in this case though as the SB-800 was not setup as a CLS remote flash but as a dumb optical slave in SU-4 mode, which will fire upon seeing any light, including the infrared signal from the ST-E2. The light from the SB-800 is then seen by the Canon flashes. Because the light from the SB-800 cannot be perfectly in-sync with the ST-E2 pulse in terms of timing and duration, the Canon flashes will see extraneous signals and get confused.

To test out the theory, I decided to run a test. Since I don’t have any Canon camera or flash, I used an all Nikon setup. A Nikon D200 was setup with its pop-up flash as master commander unit. The master output was set to “–” (off), group A was TTL, group B was “–” (off). A SB-800 in SU-4 mode with a manual power of 1/64 and A SB-600 as Group A remote flash were used in the test. If the above theory is correct I’d expect to see the same problem with this setup, and sure enough, with both flashes turned on, the SB-600 flash didn’t fire when the shutter was open. The SB-600 also made some rapid beeps indicating errors.

To see exactly what happened, I captured the pulses with a simple setup described here. The following screenshot shows the pulses captured first with both flashes off (A) then with both flashes turned on (B). The values on the top are time with unit of second.

The two captures look roughly the same. Upon close examination, however, there are clearly some differences. The following showed zoomed-in views of the each pulse groups (1-6).

Pulse group #1: Remote setting. Red arrow marks the pulse affected by the SB-800 in SU-4 mode.

Pulse group #2: Group A pre-flash. Red arrow marks the pulse affected by the SB-800 in SU-4 mode.

Pulse group #3: Group A pre-flash trigger. No pulses affected.

Pulse group #4. Group A pre-flash high trigger. First pulse was affected.

Pulse group #5: Flash amount. Last pulse was affected.

Pulse group #6: Final flash trigger.

As expected, two factors could contribute to the problem: 1) Delay. The SB-800 appears to have a trigger delay of ~40 microseconds in SU-4 mode. 2) Flash duration. The flash duration of the SB-800 is dependent on the power. It can be as long as 1000 microseconds or more. Since the CLS pulse period is ~150 microseconds and pulse duration is typically shorter than 70 microseconds, the combination of the delay and longer flash duration from the SB-800’s flash confuses the SB-600 in remote mode.¬†Similarly, it can confuse the Canon flashes in remote mode.

I had expected to see the SB-800 fire on each of the pulses from the commander but it was not the case. It was only triggered on some of the pulses. It did fire on the final flash trigger each and every time. So it appears there is a minimal delay required before it can be triggered again.

Any solution? We have discussed this in the post titled Mixing Flash Modes. In a nutshell, RF wireless triggers can be used to trigger non-TTL flashes wirelessly for a perfect flash sync.

Posted in: Photography Lighting on April 24th, 2010. Trackback URI
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