Being unfamiliar with the Sunpak 120, I am assuming that you are talking about the strobe's auto mode and it's built in sensor.
Originally Posted by dmr
My experience with Nikon speed lights is that auto does very well.
Make sure the settings on the strobe match the camera settings, if this is done the subject the strobe sees will expose properly. Bare bulb or not.
There are several wild cards that can bite you.
First the strobe's sensor is a "reflective" meter; if the main subject is a bunch of people in black it will over expose, all white dresses means under exposure. With a bit of practice you can manipulate the camera or strobe to compensate.
The other wild card is your ability to bounce.
I regularly bounce off of things behind me, if you can put a large reflector or two above and behind yourself it will help. Bigger the better.
Bounce and/or big reflectors will mitigate what many people call flash over exposure. What you are doing is simply "bringing up the house lights". A good share of the strobe's light is simply filling in the dark areas so that the properly exposed subject will be in a nice room rather than in a black pit.
One more tip, use the widest aperture that you can, this let's the real house light "help".
I should have also said use the slowest shutter speed you can get away with.
If the groups are relatively well behaved adults, tell them that you are using a slow shutter and ask them not to wiggle. 1/30 or even 1/15 may work. This technique allows more ambient light to reach the film and will not cause over exposure of the main subject, the auto mode of the Nikon strobe's control main exposure by limiting flash duration, that is much faster than the shutter, say maybe 1/1423rd for example. The strobe can see the ambient light too.
Bare bulb is a unique way of lighting. I used to use Mazda 75 bulbs a lot. These are like harvesting the sun and could do a gym full of students in one bulb. Had to lock the shutter open on the camera and use the lens cap. At 75 feet to the first row, F22 plus a 4x ND filter. Lock the shutter open, pull the lens cap, detonate the bulb in a clamplight fixture with a switch and 9V battery then put the cap back. You have never hear so many people yell D@mn at one time after setting one of them off. I wore sunglasses and kept my eyes closed.
I now see you can get $ 85.00 each for them, they are monsters. Our historian at the Air Force Museum said they would mount these under the wings of aircraft in Korea to do recon photos from way up.
I also use them to do crowd photography off the top of a 25 story building along the Ohio river for the WEBN firworks, that attracts 200,000 people. The radio radio station paid well for those night shots with the crowd lit up and fireworks over them. They would wonder if the space ship had landed when we would set them off.
I would buy 3 or 4 of them and do a test with one or two. Chances are you will not be able to get the crowd to pose for a second image without SPF 75 sun block and free welder's hoods.
You can find a few here. I sold off my last 17 bulbs. It paid for a couple of days at Yosemite park. I was always worried a static spark would set one off. A company in Hollywood purchased mine for a FX pyro tech. They add a lot of light when you blow something up I guess.
Is there any way you can get out of doing this shoot? ;)
Seriously, it sounded only difficult until you said you might only have 5 minutes setup time. Pressure like that is bound to make you forget something and/or otherwise screw up.
Others have given you good suggestions, so I will just reiterate the ones I agree with:
No bare bulb, unless you put a diffuser in front of it and a large reflecting surface behind it. A portable reflector, or even a big sheet of white foamboard will suffice. Alternatively, a large diffuser in front of the flash. But either will cut your light by a couple of stops or more.
You really, really need a flash meter. An auto flash will read the dark background and overexpose the people. An ambient flash meter will give you the right exposure in seconds, and let you check to be sure it is even across the spread of the group.
An 18mm is a really bad lens for this. If you use it, put the fat people in the middle and the skinny ones on the ends. Or put the people you like in the middle and the ones you hate on the ends. Really. I never use more than a 35mm for groups when shooting 35mm, and even then, I try not to fill the width of the frame.
But really, consider a change of venue.
If you have ever seen the shots at the school play where the background is jet black and the kids faces are completely over exposed? That's what you get.
The only way you know who's who is from the costume. It's from using an auto sensor flash. The exact same thing will happen with a dark paneled room.
Just as eddym says.
There's nothing you can do about it as long as you're using auto flash.
Well I, for one, would be interested to see how the shot turned out using the bare bulb flash! It's easy for me to say that, as I'm not on the hook to produce the photograph. My interest stems from my high school days in the 1960s when I had a Braun Hobby wet-battery flash = shoulder battery pack, big coiled cord feeding the flash head with removable reflector. (Can I get an amen?) With the reflector off, the three inch tube was vertical and threw light the full 360 degrees. The advantage was that light bounced everywhere, filling in the shadows, so less harsh lighting than with a directional flash.
I've always wanted a Sunpak 120J just to be able to play with bare bulb again. I say, go for it! Maybe mixed with a few remote triggered small flashes, if you wish.
I like Jerry Lebens\' answer. Better to diffuse the light for more coverage that\'s more flattering, than to shoot barebulb (autocorrect changed that to \"barefoot\").