I use a Nikon D90 in a no/low light situation for concerts where flash i strictly forbidden. The issue is that the AF assist illuminator light is way to white and bright to be used in this circumstance. It just to closely resembles a flash and will get be booted and blacklisted from certain venues. So the question is this. Is there an accessory or workaround for improving the ability of the cameras auto focus without using the bright white light that is the assist lamp? It looks like the speed light accessory have a red autofocus lamp but that you would have to shoot with a flash to use it? What about a gel pack, or similar over the existing lamp?
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I agree with everything Eclipse said. Also in general, the center AF sensor is the most reliable in low light. Select your AF area mode to dynamic area and use the rear selector pad to select the center sensor. Set AF to AF-S. Use the focus-recompose technique, which should be in your manual. Aim the center sensor to a point with contrast or distinct detail, and focus-recompose. Otherwise, you could always revert to manual focus, though it's tricky on crop DSLRs with their tiny viewfinders.
It sounds like you need to turn off the AF-assist illuminator.Check you manual (page 174) and you'll find Custom Setting A3 should allow you to turn it off. Be aware this will actually rob the AF sensor of contrast which is something the system depends on for autofocus. With any DSLR, faster lenses (bigger aperture, smaller f-number) usually provide the best AF performance. You haven't mentioned which lenses you are using but I would hazard a guess that something with a maximum aperture of f/2.8 or better would help. A variable focus lens with a max aperture of f4 may tend to be slow when trying to autofocus in low light.
As previous poster stated, faster lens may help. The AF assist light is very annoying, thus it is best to disable it in concert venues. Another thing you can do is focus on a contrasting edge that has enough light that is the same distance as your subject. You can try focus on eyes or eye glasses. You need to set your camera so the focus zone is fixed (usually the center zone) and lock the zone using the lever switch just below and to the left of your 4-way selector (page 56 in your manual). The lock prevents the 4-way selector from moving the focus zone. Then, set your auto focus function to AF-S (page 54 in your manual) to lock your focus when you depress the shutter half way. After you lock your focus, you can compose and shoot. I've used this technique shooting stage shows and dance venues with the given lighting. Be sure to set your white balance. Usually, tungsten or use the Kelvin setting to get the white balance right. Shooting in manual mode is helpful. Stage event lighting is tricky for an auto exposure system to get right. Set your LCD monitor to highlight mode so you can see overexposed areas (page 163).
