Trivial lighting question.
 

Just trying to remember if when shooting with a 3 point system, the house lights (practicals) are always kept off (especially when they wont be in frame) during the initial exposure, or can they be left on?

Also, just out of interests sake, why are do we adjust our exposure to just the Key-light and not all three striked at the same time?

It's not as trivial as you might think, and the answer is, as always, "it depends".

If the house lights make your shot look better, then leave them on. If not, then turn them off. There is no hard and fast rule about this.

As for exposure, I usually set the final exposure with all the lamps lit.

Yeah, the room lights can sometimes provide a nice fill. It does all depend on the look you are going for. If you are using tungsten lights, the color temperatures will be off - which is often a good reason to take them out. Also just for control.

I, too, expose with all the lights on.

Good luck.

I've always been told to set the exposure to just the key light, but always found that a bit odd.

It is good advice. Certainly the key is the most important, and you want to set your general exposure based on how the key plays on the face.

If the exposure of the key is set correctly (and you don't know it yet) and you add the kicker and that causes some zebra, then the correct answer is not to adjust the exposure, but rather to adjust the intensity of the kicker down. If you just closed the iris a stop to prevent the zebra, then what you are doing is making the face too dark. That's why they say it, and like I said it is good advice.

That in mind, it may just be laziness that I adjust based on the full lighting setup, but I pay very close attention to what the key is doing, and make sure I adjust the others accordingly to model the face the way I like it.

Why? The key light is defined as the brightest light hitting the subject of the shot, not by it's position. "Key" is the key term. All other lights work to support that light and the scene it creates. Of course you expose for the key, you have no other choice or else it's not the key.

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