If I set my D300S to "slow sync" does that control either the built in flash on the camera OR the SB800, whichever us used? I have to take some bright sunlight pictures and need a little fill flash. It sure would be easier to use the built in flash for this small amount of flash. Coleman Locke Wharton, Texas
Yes, slow sync will control either the built in flash of an accessory speedlight in the hotshoe.
Why are you using slow sync in bright light? Slow sync lowers the shutter speed to slower than 1/60s, meaning you really risk over exposure of the ambient light and the flash exposure may result in lost highlights.
Just use regular flash sync of TTL-BL and you should be okay. If in doubt, and you just need a little fill, the pop up should be okay. (In that scenario, I use Program mode and the on-board flash with my film camera or D700; on my F100, I use an accessory speedlight - SB28 or SB30)
Anthony is right. Slow sync is intended to be used in low light conditions to allow more ambient light than normal sync would provide. During bright conditions normal sync and TTL-BL should work well. Note that you can also include some flash exposure compensation to either increase or decrease the amount of fill that the camera would normally provide.
There is one more case to consider if you have an SB600, SB700, SB800 or SB900 external flash (and probably a couple of other recent models): If you want to use large apertures in bright light (and, consequently, high shutter speeds) to limit the depth of field you might want to use Auto FP High Speed sync which will allow you to use the flash at shutter speeds much higher than the 1/250 normal maximum flash sync speed.
Thanks to both of you for your great advice and explanations of the slow sync. I was under the impression that slow sync would act as a "fill" flash. I didn't realize that the shutter speed would slow.
We did the shoot today and just stayed with TTL-BL and it worked out fine. We used the pop up flash (didn't want to have to put on the SB800) and it worked perfectly. We were photographing a subject wearing a hat during the middle of the day with a high sun and just needed to take the shadow from the hat off his face.
Tom Boné, in a former life was photographer in the Marine Corp and when he retired from that, he worked for a local paper near a military base and had to shoot lots of press events, outside with base personnel wearing caps and visors. One tip he had was to rotate the camera upside down so that the flash would fire up and under the visor. Takes some practice, but it will always work.