I finally got around to exploring the CLS preflash sequence in a bit more depth. The following pair of images and explanation are my latest effort.
Two D2X cameras were used, separated by about ten feet in a darkened room. The subject D2X was on a tripod with an SB800 mounted in the hot shoe. Beside it was a light stand with three more SB800s clamped to the column and aligned with the on-camera unit. This camera was set for 1/30 sec shutter time and rear curtain synch for the SB800 speedlights, to provide an interval between the preflash and image capture sequences. The shutter delay was set for ten seconds.
The second D2X (capture camera) was on a tripod with the a 14-24 mm F/2.8 lens. Its shutter time was set to three seconds. To capture the images, I pressed the shutter on the subject camera, starting the ten second timer. I walked to the capture camera and watched the red timing light on the subject camera. When it turned steady, I pressed the shutter on the capture camera and started rotating it through a ninety degree or so arc. With a few practice tries, I could capture the entire preflash and image capture pulse sequence in the center of the capture camera frame. You will notice slight timing variations in the images as I cannot guarantee that my rotational velocity was constant.
This image shows all four SB800s in the CLS TTL mode at 0.0 exposure compensation. Under column "M" (master) the on-camera SB800 emits a series of pulses and calculates an exposure compensation value for itself. Then, under column "A" (group A) it exchanges a series of pulses with the Group A SB800 and sends it an exposure compensation value. This process is then repeated for groups B and C. During this interval, the shutter remains closed as indicated at the bottom of the chart.
The shutter opens at the left edge of the "All" column. Just prior to the end of the shutter interval, the on-camera master emits a low power triggering pulse. A short interval later, all four units fire in sync for image capture. Then the shutter closes.
Two changes were made in this sequence. The on-camera master was set to OFF (---) and the Group B SB800 was set to manual mode at the M1/2 power level.
Under the left-most column "B", the on-camera master emits a pulse that delivers the M1/2 power level to the Group B SB800. This is a one way transmission, as there will be no reading of reflected light and exposure compensation calculation. In other tests, it appears that the Manual mode power levels are always sent prior to any TTL exchange and exposure compensation calculations.
Following this, the Group "A" and "C" preflash sequences are exchanged as in the Top Image.
AT the left edge of the "All" column, the shutter opens. Just prior to the end of the shutter interval, the on-camera master emits a low power triggering pulse. A short interval later, the Group "A", "B" and "C" SB800s fire in sync for image capture. Notice that the on-camera master SB800 does not fire in synch with the other units, as it has been set to OFF (---). Then the shutter closes.
Without a four channel recording oscilloscope, I am unable to decode the preflash pulses in any greater depth. Earlier work with Nikonian Alson in Holland disclosed that some form of amplitude/frequency modulation coding is apparently employed for the preflash sequence, which must contain channel, group and exposure compensation data at a minimum. My current, ancient oscilloscope is a single channel, non-recording model. My NAS keeps overpowering my motivation to acquire such an instrument.
I suspect, but cannot prove, that the CLS technology makes some kind of negative exposure compensation calculation for multiple remote units to allow for overlapping coverage of the subject. Only Nikon knows, and they never reveal things like this.
Hope this helps take a bit of mystery out of the CLS preflash sequence.
Comments and questions welcome!
More later ... perhaps.
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