Best way to light this situation?

[filemanager]

I'm doing a shoot next week at sundown... we have a waterfall and the clients will be in front of (or maybe even on top of) the waterfall, with the sunset in the background.

I've found in my previous testing that a shutter speed of 1/6 makes the water as silky as it will get during the day - but I'm not sure about sunset, I am doing some re-testing tonight.

Anyway, should I use a flash to light up the waterfall+models, yet leave the sunset in the background darker? And if I use my flash should I use the diffuser (Gary Fong)? Or would it be more wise to use a slower shutter speed and no flash?

[i speak in math]

The Gary Phong defuser won't do anything outdoors since there is nothing for the light to bounce off of. The sunset is likely going to be very bright. You will need either a large aperture of really fast shutter to keep it from overexposing. This poses several problems.

With the sunset properly exposed (really, underexposed by a stop or two) your foreground, the water fall and family, will be pitch black underexposed. You need to use some light to bring their exposure closer to that of the exposure of the sunset. You need to compress the range of light so that your camera can capture it all.

With large aperture, you will decrease the usefulness of your lighting. You will need a lot of power to overcome f/16 or so. Studio strobes with battery packs would give you enough light. Powerful hotshoe flashes, even at full power, are probably f/8-f/12 or so.

With faster shutter, you will have to use high speed sync on your flash. This decreases the power of your flash. Your proposed 1/6th for silky water will overexpose the background way too much unless you are at, like, f/32. You will probably want to stick to your sync speed (1/250 for canon, 1/180 for pentax and varies for nikon)

Its a tricky shoot for sure.

[RichardTaylor]

Why not expose for each part of the scene individually (possibly with fill in flash for the people) and composite the exposures in post processing?