In the Part I of the Nikon CLS Advanced Wireless Lighting series, I described the very simple and inexpensive setup I used to capture Nikon CLS wireless flash sequences and showed you some captures of the simplest form of wireless flash communication. I then detoured to talk about some trivias about the Nikon non-wireless i-TTL flash process. Going back to the series again, I will talk about the flash sequences with remote flash(s) set to TTL mode.
All the tests below were conducted using the built-in flash on a Nikon D200 in Commander Mode. The flash sync mode was set to Front Curtain. The exposure mode doesn’t really affect the results if the shutter speed stays within the sync speed limit of the camera, which is 1/250s. The sequences are slightly different in Rear Curtain flash sync mode with shutter speed slower than 1/2 second. I will get to it later.
The first example was made with the D200 built-in flash in Commander Mode, Ch. 4, M: Off, Group A: TTL. There was no actual remote flash.
I labeled each of the pulse groups (blue lines). The first group is the remote setting command, which tells each remote group (in this case only one group) which mode it should be in. The available modes are Off, TTL, AA, and M. The second pulse group is the pre-flash command, which tells a specific remote group to get ready to fire a pre-flash. After that, the master unit sends out a pre-flash trigger low signal (the 3rd pulse group). If the remote flash unit detects the command, it fires the pre-flash after a short delay. Since there is no actual remote group in the test, there is no pre-flash from the remote unit captured. But I will show you examples later with remote flash actually turned on. If the camera finds that the pre-flash too weak, it will ask for a 2nd pre-flash from the same remote group. That is the 4th pulse group labeled as preflash trigger high. The remote unit should then fire a stronger pre-flash. The intensity of the 2nd pre-flash is pre-programmed in the flash. There is no communication between the master and the remote on how strong the 2nd flash needs to be. The 5th pulse group contains the flash output amount data the camera calculated for the remote flash. After a relatively long delay, the camera instructs the master flash to send the final flash trigger (the 6th pulse). If there were actual remote flash, it would emit the main flash in sync with the trigger signal. The possible reason for the long delay is to let the flashes recharge after the communications just in case the final output requires full power.
The following graphs show the zoomed-in views of the pulses.
If you compare the remote setting, pre-flash CMD, and remote flash output amount pulse groups, you will find that the first three of them are the same. That is the indicator for the channel (Ch. 4 in this test). By sending out each command with the channel information, Nikon CLS wireless flash system allows photographers to avoid interference between flashes by using different channels at the same location.
The following is an example with two remote groups A and B. Master was still set to off in the test. No actual remote flash.
Compared to the first example, the difference is the extra three pulse groups for instructing remote group B to do the pre-flash. The next graph shows two captures: the top one is the same as the one above, the bottom one is with two remote flashes for group A and B turned on but pointed away from Camera’s field of view. You can see peaks immediately after the pre-flash trigger pulses.
Well, if you have gone so far I hope you have not been bored to death. Knowing those boring details probably won’t help you make better photos. It is just for the curious mind.
Keywords: Advanced Wireless Flash, Creative Lighting System, Flash photography, Nikon, Nikon CLS