Oh, Man! If you've got an SB-800, you've got it made! Unlike the built-in flash, it will be high enough away from the lens that the end of the lens won't cast a shadow. Its a perfect setup. One of the truly stand-out features of Nikon cameras is their absolute magic use of fill flash. You just about can't go wrong.
Seriously, even if you're not too familiar with it, try to take an hour this afternoon to familiarize yourself with the settings. It will be well worth it.
Here's what you do:
- Make sure your camera is set to matrix metering with "zero" exposure compensation. You set those by holding down the buttons immediately behind the shutter button while turning the rear control wheel.
- Mount up the SB-800.
- Press and hold the little "Flash" button that's on the front of the camera, on the left hand side (as you hold the camera that is).
- With the flash button held down, turn the front control wheel back and forth. You'll see the top display go from "+1" to "-3" (those are measured in "stops") in 1/3 stop increments. Personally, in a situation like yours I prefer to set the fill flash to "-.3" (ie, minus one-third stop).
- With the flash button still held down, turn the rear control wheel. You'll see the top display switch between "eyeball" and "rear". Personally, I always use "rear".
- Make sure the flash is set to "TTL" (stands for "through the lens") mode. I have an SB-700, which has a slider switch for that. I think the SB-800 has it in the menus.
That's it. Prepare yourself for portraits that just "POP"!