Last weekend, I was shooting an event for a friend at a nighttime party with very low light. I had D700 / 24-85 2.8-f, SB900 (mounted on the camera). I couldn't get the lenses to focus in auto and couldn't see well enough to manually focus. As far as I can tell, all equipment is functioning perfectly. People at the party had point-and-shoot cameras that were working fine. Camera was able to focus properly, when I shot in the banquet hall, where light was ok. But as soon as I went outside to shoot cake cutting, could not capture many important moments. Normally, my camera is able to auto-focus in totally dark room with the AF assist light only. But don't know what happened that day. My camera setting were - Single Shot, Manually selected center focus point, 50mm around f/3.8, Sb900 on camera hot shoe pointing straight on to the couple cutting cake, and then kissing, and then a group shot with friends. Nopes. Camera won't focus. I missed a lot of important shots, which was very frustrating. I am glad it wasn't a paid gig or a wedding. Any help would be appreciated.