This is an ambient light situation, set your meter accordingly - ie either manual or Aperature priority. F stop you want determines your ISO or rather lets say the max noise level your camera generates determines your highest ISO which in turn determines the shutter speed n f stop combination. What combination is chosen is what can you shoot without bluring, what fs top is best to use is determine in part by the focal length you use. The effect you want also plays a part. You can see that there is no one true answer - a truthful one is it depends. Then you have variations what if you shoot backlit with front fill, with off camera rim lights, with video lights........ nothing is cast in stone.
These are things that a working professional needs to know and consider in the approach and execution of a shooting sequence. Yah it's technical but thats part of the biz.
man, i encountered this situation once, it turned out just like a black and white pic, with the spot light on and the rest is total darkness, seriously i hope to see an exact pic of how others do it. how you turn this difficult situation into a stunning pic.
As ALWAYS advised to many ... UNDERSTAND LIGHT ...
This was one of the slightly tougher shoots I did ...
Basically, MEASURE all the light ratios, preferably, before hand during REHEARSALS.
Then balance out available/existing light and flash.
NOTE: MANY, MANY compromises would have to be made along the way.
BTW, in this event, the spot light were those kinds that had a MINIMUM throw distance of more than 50-100 feet (err... think of 2 or 3 storey high >50 feet from the stage super-intense spot-lights) ... and was OVER-POWERING the short (less than 45 feet) SAM cathedral 'catwalk'. OR in short, the minimum power of those spot-lights were something like 15-50 stops over ambient.
lol..seen some really crappy words from some folks. Talk so much, but not saying anything useful
I use to shoot weddings and i hve this question many years back when i started
actually, it's very simple.
Depending on what u want, here's what you do
1. Couple bright and the rest dark.
Simply off ur flash, put to aperture mode and shot ( making sure exposure is for the bright light, ie, use center weighted metering or spot metering. If u are in average or evaluative, pic will be usually OVERexposed and might get blur=> ask yourselve why it's gonna be overexposed or blur...If u can find out the answer , u know photography basics better than a lot of the crappy folks here.)
2. Couple and audience all bright
This is usually more tricky and the pic effect usually not as good as 1.
Assuming u have a powerful flash, point flash towards ceiling, at manual mode and shoot.
Not advisable if ceiling is tall , dark coloured or if ur flash is weak.
3. Of course, intermediate are some other effects that require more skill. Eg , only shilleoute with only some glint of birghtness. Very nice for Black and white shots ( esp with entrace smoke). This depends a lot on experience to gauage the amt of compensation to dial in.
For couple march in, do not set to AF tracking and large aperture. Tat's another newbie mistake. Unless u happend to have a 1D of D3 kinda camera AF, most cameras cannot track ( esp moving TOWARDS u kind of motion which is very different from moving ACROSS kind of motion) very well in low light.
Set aperture to around F5.6 or at least F4. This gives a larger focal plane of fcous ( again, if u understand what this is, u know a lot more), off multi AF and use single spot. Best and effective way.
for mod DM samples are not so tricky, at least he can fire from a spot without moving himself. sory DM hehe....
but when I shoot couple marching during wedding banquet, I'm moving myself, usually I shot around at least 5 to 10 shots from the time the door open till the couple up the stage, and I make sure each shots is from different angles, some high some low some from left some from right and some at their back. I use M mode on camera and TTL bounce flash, once the spot light is shine on the door I check my meter make sure it didn't overexpose, so once the couple march in I will start shooting, even till the flash didn't fire due to slow recharge I still able to get usable shots.