Forget shutter speed; don't go above the camera's maximum sync speed (is that 1/250th or 1/500th on a 1D4?) but just set it to control the actual ambient, i.e. the amount of street lights (or whatever). Put it high, say 1/250th, to cut out all the ambient or go long, say 1/30th, to let it burn in a bit.
Set the aperture to sort the flash exposure - I'd start with the flash on about 1/8th and just tweak the aperture until it looks right. ISO 400 -800 should be fine.
Are you using the flash off-camera or on the camera hotshoe? If it's the later then it may pay to set the flash to TL, because subject to flash distance will b changing a lot and doing it manually could mean a lo of faffing.
If the flash stays in one place then set it manual and you can just move around freely.
Fishing at dawn by
Pat MacInnes, on Flickr
This shot was done with off-camera flash (click the image to see the settings) so all settings just stay constant as I move around. This was about 8am, as the light was starting to come through but just imagine that if the light levels were much lower then the background would be much darker. Exposure for the ambient is 1/80th and the aperture was f/4. ISO was 800.
If i doubt, head over to
www.strobist.com and go into the strobist: 101 section to find out more about flash....
