Philip Copley

There are three issues:

 

One, you can easily print down or darken in post a shot if you want so it's better to avoid underexposure unless you absolutely need it for the shot (for example, a moonlit room would naturally be underexposed.)

 

Second, a low-key scene doesn't necessarily mean an underexposed scene. It just means a scene that is predominantly made up of dark areas. But the small areas that are lit may be a normal exposure if not even overexposed. You look at the temple scene at the end of "Apocalypse Now" and while some shots are overall underexposed, many have the figure in a spotlight at full exposure, coming from one side to keep half the face dark, but surrounded by darkness.

 

In terms of how underexposed the shadow side should be, that just depends on what you want. There's no law saying that you have to have ANY shadow detail. But generally, about three stops underexposed is a good idea if you want some detail but have it look dim in the shadows. Maybe four stops underexposed if it's low-con negative transferred straight to video. But for printed material, generally no darker than three stops under if you want detail in the shadows.

 

But is also depends on the reflectance of the subject (if you were metering with an incident meter.) A dark-skinned person may need more light in the shadow side for detail but a fair-skinned person would need much less.

 

Third issue: don't judge how much to underexposed based on a video transfer. You might compensate by underexposing more and more, and when you finally make a print, discover that your negative is not dense enough to strike a print with good blacks. You don't know if your dailies come back and the face looks light if you did that or the telecine colorist did that, so don't assume that you need to underexpose more.

Does that mean You underexpose the darkness side 3 stops down from the reading on the face or 3 stops down from the reading on the lit side.

Sorry if this is a stupid question.