Let's assume we have these initial conditions:
normal indoor ambient lighting, camera 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 preflash sequence.
2. The flash metering system measures the reflected light in the center of the frame from the preflashes and determines the power for the main flash.
3. The calculated power for the main flash is then modified by flash compensation that is 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 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 commuication takes place during the preflash sequence. Here are the steps:
1. The Commander fires a control flash sequence that tells each of the remote flashes to fire its preflash sequence.
2. Each remote group fires its preflash sequence one at a time. If there are multiple flashes in a group, they will all fire their preflash sequence simultaneously.
3. Based on some very valuable work by Hal Becker (my fellow Moderator 'HBB' at Nikonians Speelight Forum http://www.Nikonians.org/forums), 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 as smart as 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.
A power setting is also set into the commander unit if it contains a flash as well. Multiple flashes in the same group will all receive the same power settings.
4. The shutter opens.
5. The Commander fires another control sequence telling all groups to fire at the power decided in step 3.
Also, the preflash sequence gets visibly longer when more remote groups are being controlled, and it becomes more important to block the visible light from the commander to avoid the preflash causing the subjects to blink.
If you want to see the preflash separately from the main flash, use the FV Lock button. You will see that when the flash is in the hot shoe, the preflash sequence is very short. As soon as you use the flash wirelessly, however, the preflash sequence gets visibly long due to all the extra communication that must take place in this mode.
Also the preflash sequence delay can cause problems when shooting action, so it is best to use FV Lock first which removes the preflash delay.
Next: How to Shoot Large Groups using Nikon CLS