David Mullen ASC
Moderator
You're still not getting it. What if I wanted a lit face against a pure black background? That black background could be twenty-stops underexposed for all I know. It doesn't matter -- I want it to be black. It may take up 90% of the frame, who knows.
But what if I want the background to be four-stops under, based on testing to know just how dark four-stops under will look? Then I will expose it to be four-stops under. It will only go noisy or grainy if either (1) the film stock or camera is normally noisy or grainy at that ASA rating, in which case I may want a slightly slower rating or slower film stock and (2) if I change my mind in post and decide it looks too dark and try lifting it -- then it will get noisy or grainy.
But otherwise, the overall noise is really based on how you print or color-correct the image. If only 10% of the frame was a lit face against a background that is 90% underexposed by four-stops, and I picked an ASA rating that gives me an overall noise level I like, then I should be fine if I exposed correctly. But let's say that I think the face looks too dark and I bring it up one-stop -- now the four-stops underexposed background really looks more like it was five-stops underexposed because essentially I accidentally underexposed the whole frame by one-stop. And now the image may look noisy.
But if you want a pure black background for a scene, then you can put a pure black background. It's not how dark the background is in this case, in terms of how noisy the blacks are, it's how you expose the subject and whether you end up lightening or darkening the subject in post. If you expose correctly, you may be fine. If you overexposed slightly and brought the image down, the blacks may even look better, but as you overexpose more in digital, you get clipping, which is ugly. So you are limited in terms of overexposure mistakes too, just like underexposure mistakes.
So the key is to expose correctly for the look you want based on the ASA rating you think looks best generally. And because it's digital, you should be able to expose correctly because you can see the results right there on the set.
But what if I want the background to be four-stops under, based on testing to know just how dark four-stops under will look? Then I will expose it to be four-stops under. It will only go noisy or grainy if either (1) the film stock or camera is normally noisy or grainy at that ASA rating, in which case I may want a slightly slower rating or slower film stock and (2) if I change my mind in post and decide it looks too dark and try lifting it -- then it will get noisy or grainy.
But otherwise, the overall noise is really based on how you print or color-correct the image. If only 10% of the frame was a lit face against a background that is 90% underexposed by four-stops, and I picked an ASA rating that gives me an overall noise level I like, then I should be fine if I exposed correctly. But let's say that I think the face looks too dark and I bring it up one-stop -- now the four-stops underexposed background really looks more like it was five-stops underexposed because essentially I accidentally underexposed the whole frame by one-stop. And now the image may look noisy.
But if you want a pure black background for a scene, then you can put a pure black background. It's not how dark the background is in this case, in terms of how noisy the blacks are, it's how you expose the subject and whether you end up lightening or darkening the subject in post. If you expose correctly, you may be fine. If you overexposed slightly and brought the image down, the blacks may even look better, but as you overexpose more in digital, you get clipping, which is ugly. So you are limited in terms of overexposure mistakes too, just like underexposure mistakes.
So the key is to expose correctly for the look you want based on the ASA rating you think looks best generally. And because it's digital, you should be able to expose correctly because you can see the results right there on the set.