Don't fret. You are not out of the woods yet. Lighting is an extremely important element of filmmaking, and no decision regarding it should be taken lightly. However, the shoot must go on. Any DP (that's Director of Photography, although some prefer the title cinematographer) worth their weight in gold knows a few alternatives.
First of all, determine what medium you are filming on. If you are using digital, congrats! This will be even easier for you. Film is a little harder and more costly when you have to reshoot, but it is not impossible to guesstimate your lighting needs.
If you are using digital, I am assuming you have a monitor on-set. Take two crew members of various skin tones--preferably a white crew member and a black crew member--and place them on your main actors' "position 1s" (the first place the actors will be at the start of shooting). Instruct them to move around the scene. Then ask them to trade places in order to visualize how a particular lighting scheme will fall on your main actors.
Things to look out for:
- "Raccoon eyes." Deep shadows over the eyes, unless that's what you're going for.
- Deep shadows over faces, such as noses.
- Hair highlights.
- "film skin tone" matches "real skin tone."
- Actors moving from light to shadow.
- Hard background shadows and areas that lack definition.
If your DP knows the Zone System (and he/she should), this method will be able to help determine the lighting needs. But here is a hint, if the camera looks great on your redheaded PA, but 2 t-stops under with your milk chocolate-colored grip, and you have a "black Irish" actor, drop 3/4-1 1/2 stops, depending on the actual skin tone color and the mood of the scene. It is always better to a little underexposed than overexposed. You start to lose detail when the frame is blown out. Underexposure is easier to fix in post, using AVID or FCP (Final Cut Pro).
Remember not to rush this decision. Take into account:
- Color temperature of your lights
- The mood of the scene
- The actual position of some lights
- Any natural or source lighting (you know, the sun or your lamp in frame)
- Wattage (for safety reasons)
- Your ISO ("film" speed, even in digital)
- Your shutter-speed (usually at 1/60, but could be more or less depending on your situation)
If you are using a film camera, make sure you bring an SLR (Single-lens reflex camera) on set. It can be either digital or film, but preferably digital for instant results. If someone should have one, it should be either the script supervisor (continuity) or the art department (for set dressing reasons). SLRs usually have meters. F-stops equal T-stops in the lay sense. Take several stills of your actor and notice what the meter reads each time. ISOs are different in SLRs than cinema cameras, but if your DP is any good, she/he will know how to adjust. A conversion chart is here.
If this fails or you are overly nervous about lighting, you can always send a PA to a camera store to rent a meter. One of my favorites is called Freestyle, located on Sunset Blvd in LA. Make sure to get that student discount...but you didn't hear that from me.