Shooting a large group, say 40 people or more, using only two Nikon SB-800s in umbrellas is a challenge, however, you can get decent results if you do it right. SB-800s are fairly low power flashes for this type of work, so placement of the equipment is crucial. Here is an example of a recent group shot I took with just this equipment:
Notice that this image is far from perfect, but it is acceptable. It clearly shows some of the standard problems you usually run into with a large group and small flashes.
I used my camera in Manual mode and the flashes in TTL mode. I used the pop-up flash as the Commander with a Nikon SG-3IR filter in front of it. The SB-800 only responds to IR light that enters through the small round red window, and since the pop-up Commander puts out both visible and IR light, the SG-3IR filter is useful to suppress the visible portion of the light to help keep people from blinking.
In a large room like this, where there are no walls or ceiling from which the pre-flash pulses can reflect, the SB-800 flashes in umbrellas must have their round red IR windows facing the Commander, or the pre-flash pulses from the Commander will not control or fire them.
As you may be able to tell, the ambient light was strong, and it was some weird mercury vapor color so there was no way to match the flash light to it. Therefore, there is a slight color shift between the front row and the back row indicating that the front row was lit mainly by the flash, but the farther away from the flash you get the more the ambient had an effect.
Holding focus from the front row to the back row is always a challenge. You address this by creating as much depth of field as possible by using as small an f/ stop as possible. However, the smaller the f/ stop you use, the more power is needed from the flashes to provide enough light.
You also want to have the rows stand as close together as possible. Notice how closely the rows are standing to each other in this image.
I used f/5.6 on this shot, because that was the smallest aperture I could use and still get adequate lighting from my flashes. I set the camera in Continuous Servo AF with the switch on the front of the camera, CSM a1 to FPS Rate (i.e., Release Priority), and CSM a6 AF-ON Only (i.e. to focus only when the AF-ON button is pressed). This allows me to aim at the second row, push the AF-ON button to focus (or use manual focus if I want to), and then recompose to place the group in the frame as I want it, and then push the shutter button to take the shot, and the green focus light doesn't have to be on for the shutter to release. This shifts the focus toward the middle of the group so depth of field is optimum for getting everyone sharp. f/5.6 is barely adequate for holding focus across a group this large. f/8 would have been more desirable if I were using say three or four SB-800's.
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