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There's a certain amount of light in this room right now that's illuminating me this much. If we double the amount of light, if we bring them twice as many of the exact same kind of lights and turn them all on, our eyes will not actually register a doubling of illumination. Our senses don't work that way. All of our senses are that way. If I hand you a bowling ball and then hand you another bowling ball, you don't actually perceive a doubling in weight. Our senses are nonlinear; they actually look on a logarithmic scale. Film is the same way. If you double the amount of light in a scene, you don't get a doubling of illumination when you're shooting film.
Your digital camera, though, is different; it employs a linear capture system. What that means, practically, is that when you're working in low light there is an exposure strategy that you can employ that may help you keep noise down. Here is how it works. There are a certain number of levels of brightness that your camera can capture. Say it can capture 4000 levels of brightness. Half of those levels go into representing the brightest stop in your scene. In other words 2000 of those 4000 available shades of gray that it has go into just the brightest stop.
Half of what's left over from that remaining 2000 go into the next brightest stop, half of what's left over from that into the next, and so on and so forth. What that means is when you get down to the darkest tones in your image, they may only be represented by four or eight shades of gray, or four or eight tones or values. So in other words, your camera is really good at capturing bright things but not so good at capturing dark things. Most of the time this doesn't matter. Most of the time your RAW converter does a fine job of equalizing everything and evening everything out.
If you're working in low light though, underexposing or capturing lots of dark stuff, it means you're putting all of your data into those lower stops where the camera simply doesn't store that much information. You're not really playing to the camera strengths. Very often, therefore, when you're working in extremely low light, if you overexpose your image--not overexposed to the point of blowing your highlights out beyond recovery, but exposing so that all of the data in your image is more to the right of the histogram--then you've got a really data-rich area that you can push down into the shadow areas.
In other words, you can darken your image in post-processing, push all of that data down into the shadow areas, and end up with shadows that are possibly less noisy than if you had exposed normally. Let me give you an example. I can't show you the full noise response here because it's very difficult to represent that on a small screen, but here's a case where I did come in and change my exposure strategy. This was my initial exposure as I calculated it, based on the camera's metering, and you can see all of my tones are clustered here at the left end of the histogram, but the really value-rich tones would be over here on the your right end of the histogram.
By cranking up my exposure, I can capture data into the brighter areas of the histogram, and here you can see, I went up to here. Now really, to have done this right, I should have exposed even more, cranked up my exposure more, so that more tones come all the way over here. But I'll be honest with you, it was really cold there and I wanted to go home. Now I'm always running the threat of overexposing bright highlights in my image, but if I remember that I've got highlight recovery capability built into my RAW conversion, I can push all the way over into having a little bit of spike over here and be able to recover that and really darken my data back down into those shadow areas and greatly reduce noise.
This means that out of the camera, my images are going to look awful. They are all going to look way too bright, they're going to be overexposed, possibly have lost some highlight detail, but once I start processing them, they're going to really firm up and possibly exhibit much cleaner shadows. We're going to be exposing this way on some of our shots so you're going to get a little more experience with how this works and how to use the histogram on the back of your camera to keep track of this while you're working. Later, we'll process some of these images.
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