Cannibalized, I'm a little late to the clambake, and I've not done a LOT of concert shooting, but check out my galleries here:
http://www.hikariphoto.com/proofsets/metal/
http://www.hikariphoto.com/proofsets/esteban-carlos/
The first gallery is a heavy metal concert I shot as a favor for a friend who is a promoter. The second was a hired event; the artists are Esteban (the guitar player) and R. Carlos Nakai (the native American flute player), fairly big names. I'm not name-dropping; I had no idea who was performing when I was hired for that job and it scared the hell out of me when I found out. Esteban's quite a nice guy though.
For me, a HUGE part of concert shooting is the ambiance. Using flash in my opinion eliminates the ambient light, flattens everything out, and casts nasty shadows behind whoever is in front. The light falloff from a direct flash -- it's not pretty. Not to mention you run the risk of distracting (and thereby pissing off) the performers, which makes you unpopular at best. Unless you can get in and preset your speedlights on remote controls or something, where they will evenly light the stage and the performers, you're not going to get particularly good results with a flash, on or off a bracket. FWIW, I do not and will never use a bracket. Big, bulky, clumsy, heavy, and they don't move the light off the camera enough to make a difference. But then I've been learning flash photography from the best in the business, so I've picked up a few techniques here and there. Also, contrary to what someone else said, the flash combined with the long lens actually
increases your chance of red-eye. I threw away SO many wedding reception images before I figured that one out.
Anyway... in concert photography, especially if you are the "preferred" photographer and have the run of the venue, you have two best friends for good concert photography. First is high ISO (Nikon shooters will finally have a tolerable ISO above 800 in November). If you consistently nail your exposure, you can get away with as high as your camera will go. All of my metal concert shots are ISO 3200, and that place was a CAVE. It was painted black on the inside, flat black that absorbed every particle/wave of light except direct flash, and there was nothing to bounce off of. The Esteban/Nakai concert was outside, in the mountains of Arizona, and again, there was nothing to bounce light off of.
Your second best friend is a good high-quality monopod. It needn't be expensive: a solid Manfrotto won't cost you more than about $60. The monopod will allow you to shoot 2-3 stops below what you would normally be at. I was getting 1/20-1/60 second in the heavy metal concert, and up to about 1/160 at the Esteban/Nakai one, at f/2.8 on my 70-200 L (non-IS). The monopod also takes up a LOT less real estate than a tripod, is much more mobile, and can double as a weapon if needed -- mine got me out of an Anti-American riot in South Korea many years ago.
At any rate, the short answer is you shouldn't need the flash. Get yourself a monopod, jack up your ISO, nail your exposure, and go play. It's great fun, and since they won't know you're shooting until you show them your pictures, you'll make some friends.
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