原创 Nikon CLS Advanced Wireless Lighting, Part V

Nikon CLS Advanced Wireless Lighting is actually very capable. It handles all the functions you can do with a Nikon Speedlight attached to the camera’s hot shoe except the RPT flash. Specifically, Flash Value (FV) lock, FP high speed sync, modeling flash all work seamlessly. It even works when you use mirror-lockup function on the camera. In the part V of the series, we will look at the FV lock function.

Flash Value (FV) Lock

When you push the FV lock button (on my D200, I assign the FUNC button to FV Lock), the camera will initialize the pre-flash sequences to determine the flash output amount for all groups involved. Then you can recompose and take a photo without causing incorrect exposures for off-center subject. This is fairly straight forward in regular TTL flash mode but more complicated in wireless lighting setup due to the extra communication requirements. To illustrate the flash sequence when FV lock is used in Advanced Wireless Lighting, I use a simple example in which Master (a Nikon D200 in Commander mode) is set to TTL mode and remote group A is also set to TTL mode. I first pressed the FV lock button, then pressed the shutter release button all the way down ~1 second later.

The following chart shows the captured flash sequence and zoom-in views of the details. I marked each group of pulses with numbers from 1 to 8. The #8 pulse is the final flash output.

If you have followed the previous articles of the series, these should not be mysterious to you any more. Pulse burst #1 is the remote setting command. The command tells remote Group A to go into TTL mode. After that, the master emits two pre-flashes (#2 and #3). Pulse burst marked #4 is the Group A pre-flash command. When the remote flash in Group A sees the command it will wait for the pre-flash trigger (double pulse). When it arrives, the remote group will emit the pre-flash a short time later. Soon after that, the master sends out the flash output amount data to remote groups as shown in pulse bursts #6. In this example there is only group A. When the shutter is pressed, the master sends out the the flash output amount command again (pulse bursts #7), before sending out the final flash trigger (#8).

Why does the master send out the final flash output amount data twice, once right after the pre-flash sequences and once after the shutter release button is pressed?

I think this is a reliability enhancement design. If the master only sends out the flash amount data for remote group once after the pre-flashes are done, remote group will have to wait for an arbitrary time for the flash trigger to arrive. This will not be reliable because stray light pulse from other Speedlight or fireworks can trigger the remote flash.

What about only sending the flash output amount when the shutter released button is pressed? In this case, the remote flash is left in a state of confusion because it expects to receive further instruction from the master unit after the pre-flash. But it may not get it within a pre-configured time-out period when FV lock function is used.

By sending out the amount data twice, the above problems can be avoided. If you look carefully, the two final output amount transmissions are slightly different. The first one has command code “0011″ after the channel indicator, while the 2nd one has the usual “0101″ command code. Basically, the first one tells the remote group, “Hey, get ready to fire this amount of light but wait as long as needed and don’t dose off…” The second one says, “Hey, this time is real deal. Emit this much light. I will tell you to fire very soon…” When the final flash trigger is sent out soon after the final output amount transmission, the chance of mis-firing caused by stray light is low.

Another reason for this is as reader Matti pointed out in the comment section, the photographer may change ISO or aperture after pressing the FV lock button. The camera is smart enough to recalculate the flash output amount then sent out the new flash output amount data when the shutter release button is pressed. Again, this seems to render the first transmission of flash output amount useless but it perhaps part of the overall design to make sure the communication protocol is as robust as it can be.

Practical implication

If you have been using FV lock to prevent the closed eye problems, you may be disappointed to know that this trick may not work with wireless lighting setup if your subject has a very fast reflex.

To be continued …