Fill flash is needed when you have a serious difference in light for the background and the subject. This commonly happens in strong lighting with harsh shadows.
Unfortunately our puny cameras cannot handle such a large range of light like the human eye, so you are going to either overexpose the background or underexpose the subject. Most people would opt to overexpose the background aka blow out the background and let it become some ball of white rather than have their subject look like some dark chunk of charcoal.
One way to get around this, is to expose for the background, and then use 'fill-flash' to blast some light into the now-under exposed subject.
This helps equalize it out and lets you avoid making the trade off of "over exposed background" or "under exposed subject".
Okay, this all sounds great in theory, so how do I do it?
One way to help do this,
- Press the flash button to enable flash
- Set the flash to -1.7 or so.
- Disable matrix metering and go to spot
- Use spot metering for something in the background.
- Hold on to the AE-L button so your exposure is locked to the background.
- Fire away
You should get a "pretty well" exposed shot.
As for the flash, here is what I know of it. Hopefully someone will correct me if I am wrong.
With the SB-800 you should leave it on. You could either choose balanced fill or manual. Thom Hogan suggests avoiding balanced fill since it will not give you consistent results.
You should always use a diffuser and if you can, try to bounce it on something to diffuse the light even more. Umbrellas would be nice, but I think we are starting to push the thresholds of convenience here.
If you can, take the SB-800 off-camera to do wireless flash firing. This is the best way to reduce red-eye.
The D70 has a unique capability of triggering a SB-600/800 with it's internal flash (aka commander mode). The only way the other cameras can do it is with an SB-800 in commander mode.