I may be mistaken, but I'm don't think the SB-600/800 has an AF-assist illuminator. Regardless, the D70s does have one. If you don't want to use the SB-600/800 flash, just turn it off or remove it from the camera and the D70s should use its own AF-assist illuminator.
Once again, I was mistaken. In looking at the manual, for the SB-800 anyway, the AF-assist illuminator will come on but the flash unit will not fire when "FIRE" is set to "OFF" in the custom settings mode. See pages 63, 67 and 70 in your manual.
Press the "SEL" button for ~2 seconds to get into the custom settings mode.
ONLY use AF-assist on SB-600 / SB-800
Linear Mode
