Let's assume we have these initial conditions:
Normal indoor ambient lighting, camera in A mode at f/3.5 and ISO 400.
Flash in Hotshoe
When you shoot a flash picture with the flash in the hotshoe, the following flash sequences take place:
1. Flash fires its pre-flash sequence.
2. The flash metering system measures the reflected light in the center of the frame from the pre-flashes and determines the power required for the main flash.
3. The calculated power for the main flash is then modified by the flash compensation that may have been set on the on the camera or the flash. This includes the dedicated flash compensation button on the camera, the camera EV setting, and the compensation setting on the flash itself. The power output determination for the main flash is now finalized.
4. The shutter opens and the flash fires its main flash.
Remote Wireless Flash
Now, when the flash will be fired wirelessly a lot more communication takes place during the pre-flash sequence. Here are the steps:
1. The Commander fires a control flash sequence that tells each of the remote flashes to fire their pre-flash sequence.
2. Each remote group fires its pre-flash sequence one at a time. If there are multiple flashes in a group, they will all fire their pre-flash sequence simultaneously.
3. Based on some very valuable work by Hal Becker (my fellow Moderator HBB at the Nikonians Speedlight Forum, we now know that the flash metering system reads the reflected light from each group individually and immediately calculates and sends power information back to each group. Note that this system is not smart in the way we originally thought. Each group's power is set irrespective of other groups that may be in use, and there is no adjustment for a situation where multiple flashes hit the subject in the same spot. I believe that this is why each group alone is purposefully set to underexpose the subject. Then, when multiple flashes are used, like in a studio, the subject is usually exposed properly. So in the end it is very smart after all.
A power setting is also set into the commander flash itself as well. All other multiple flashes in the same group will receive the same power settings.
4. The shutter opens.
5. The Commander fires another control sequence telling all groups to fire at the power level decided in step 3.
Also, the pre-flash sequence gets visibly longer when more remote groups are being controlled, and it becomes more important to block the visible light from the commander flash to avoid the pre-flash causing the subjects to blink.
If you want to see the pre-flash separately from the main flash, use the FV Lock button. You will see that when the flash is in the hot shoe, the pre-flash sequence is very short. As soon as you use the flash wirelessly, however, the pre-flash sequence gets visibly longer due to all the extra communication that must take place in this mode.
Also the pre-flash sequence delay can cause problems when shooting action, so it is best to use FV Lock first which removes the pre-flash delay at shooting time.
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