I turn the AF to big white rectangle, CMS a3 51 point 3D, AF activation On, AF point illumination Auto, S mode. When I try to get focus, the view finder doesn't light up red around the focus area, only green focus light on and auto assist light in front of the camera not light on as well the auto assist of SB-900. Focus red light on if I set to Crosshair and Tit but not auto assist light. I use software A 1.0 and B 1.0. It worked before. Please help if you can, thanks
I may not be much help, but I referred to my Thom Hogan Guide to the D700 and some lenses will block that autofocus assist light. He also states that some lens hoods will block the focus light, some distances too close or too far, to allow use, and he generally recommends against using the autofocus assist light. You may have changed your focus lock on setting also, that could affect this.
If you have his book,refer to page 427. I highly recommend his book if you do not yet have it. Debra
When you use Auto Area focus you are letting the camera pick which of the 51 points it wants to set as the focus points and it will not show you which one you it is using. That is why you are not seeing them light up oin your viewfinder. If you want to see the focus point switch to dynamio area focus.
For the best results with 3D tracking, you will want to set the AF points to the dynamic area AF mode (the one that looks like a plus sign) one with 3D tracking enabled. Check page 7 on the camera manual and you will see how to set that. It is item 10 called the AF-area mode selector. Page 74 talks all about it.
Custom settings a3 allows you to pick how many AF points to use (9,21 or 51 or 51 (3D tracking). You will want to enable the latter one to get your 3D tracking. Then use the continuous AF mode.
I set my auto focus so it is on the back button and not via a half shutter click so that makes 3D tracking easier IMHO. The 3D tracking requires you to select an AF point and then focus on the subject so I just keep holding down the AF-on button on the back with my thumb and keep hitting the shutter.
You said it worked before so I know you had it working but these should be steps to back track and find out what you are missing that you had before.
As far as your question is concerned about the AF-assist light... a9 is the custom setting to enable it and it tells you to use an lens with a focal length of 24-200mm and you need to remove the lens hood.
Page 375 has all the restrictions with the AF light and AF assist only works when the lighting is considered poor by the camera and when you are in single servo af mode and that you selected the center focus point. All other focus points don't enable the AF assist light.