I have to take a picture of a choir (about 60) in a church. I have a D7000 with 18 - 200 mm lens and I also have a SB 910 speed flash.
I have my flash held above me to take a test picture and although it seems to fully light the area I am worried that it will not be enough when the choir are in position. Can anybody advise me? Do you think I will need two extra flash units either side and if so what sort?
Not sure of a solution, but you can be aware of the problem.
The issue with flash is the inverse square law... Flashes fall off with distance, and so only illuminates ONE distance properly. If the background or back row is 41% farther, the flash intensity falls off one full stop there. Two flashes at same distance will not change that rule (but will be 2x more light, allowing exposure to be stopped down one stop).
Bounce can help the range at home, but bounce is not likely possible in church auditorium (too high). Different lights to illuminate the front and also the back separately and ideally will certainly help larger spans, but is very difficult for a group or choir.
But the range can depend on the distance.
If near point is at ten feet, 41% more is 14 feet. Then if flash is adjusted for 12 feet, that is plus or minus 2 feet (and 1/2 stop either way ... which is still a pretty large difference).
If near point is at twenty feet, 41% more is 28 feet. Then if flash is adjusted for 24 feet, that is plus or minus 4 feet (and 1/2 stop either way, and 2x distance needs two stops more flash power or aperture or ISO,
but is a 2x larger range than 10 feet). Zooming can make this feasible. A problem is that a group of 60, of say four rows of choir chairs, might need 12 feet of range?
36 feet plus 41% is 51 feet, or an eleven foot range (for a one stop variation, plus or minus 1/2 stop). Be aware of depth of field, aperture should be closer to f/8 than to f/4... which two stops needs 4x more flash power or ISO. And a 2x greater distance needs 4x more flash power or ISO too... so that's 16x more power.
You might be able to judge satisfactory exposure range in practice by judging the light on the empty chairs? Or better, take three friends or kids to sit on first and last and middle row for a practice test. Measuring the depth of the rows will give you useful information.
Groups of a few rows is very often best done with both the flash and the camera up high (on a step ladder), both peering down into the rows. This height and angle makes the rows be a bit closer to equal distance for both flash and focus. But set up working with the middle range.
If we knew the width of your group, then we could compute the necessary zoom focal length for any distance, and from SB-910 guide number, could compute aperture for its maximum power. Or a trial test could determine it. Not knowing, and ignoring all that, and wildly guessing 35mm zoom, SB-910 DX guide number for ISO 400 is GN 232, divided by 41 feet (halfway between 36 and 51) is f/6.3. Two equal flashes (at maximum power) would be one more stop, or f/9. (GN calculator at
Understanding Guide Numbers, including GN Calculator ). It's an inaccurate guess of course, but maybe halfway ballpark?