Hi Mike,
Well there are a couple of reasons why I underexposed. First of all, I find most stocks way too saturated. As you overexpose, I find you increase saturation. Of course you can use soft light, or filters to reduce color saturation, but that limits you creatively in otherways.
The 2nd thing is the control of what subsequently happens to the film. Telecine and broadcast operators notice milky blacks. They may not notice other things, but milky blacks, they notice. So by underexposing, you are preventing them from printing up too much, and ruining your film.
The third reason I underexposed was to use the torch light as a key, and a fill. I love torch light, and I have used a hundred different methods to imitate, cut roscoflex, mirrors, sliced gels in front of the light, but nothing works as well as real torch light. I have found that 5218 holds up very well at the bottom end, and doesn't pick up a whole lot of grain. So I used real torches everywhere (we had some 3000 made for us). To help in the foreground, we had polished large reflectors made, from pounded thin sheet metal, cut a hole in the bottom, put a torch there, and it projected the light onto the faces of the actors. We would have one on each side of camera, a bit like an obie light.
We would suppliment the background with partially corrected HMI's, and then allow the orange, blue mix. It is a remarkable look. It was taking four weeks to get dailies back, so we had no certain idea what it would look like. About three weeks in to the waiting, I was starting to think, I had made a terrible mistake, as we all do. Turns out I was lucky. A few owws and ahhhs from the producers and director and I was home and dry. No tests were possible, we were in the north of India, there was orange dust in the air all the time, and I had a crew of 300 hundred, speaking 4 different languages, and of six different faiths. And I was mixing light sources. I must have been on crack. But it turns out to be the most beautiful thing I have ever shot. Whether thats saying much, is for you to decide. s