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Here is a weird one. If the amount of light in this room is doubled, I do not actually perceive a doubling of light. I perceive less than that. All your senses work this way actually. If someone hands me a bowling ball and then they hand me a second bowling ball, I'll probably think, "I really wish this personal will stop handling me bowling balls." Then I might actually realize that I'm not experiencing a doubling in weight. Our senses are nonlinear. That is my sense of brightness does not follow perfect doublings. My sense of brightness increases on a curve.
Now don't worry too much about understanding that. The practical upshot of it is that we end up with a very good ability to see detail in bright highlights and dark shadows. The image sensor in your camera though captures light in a linear fashion. If you double the amount of light, the image sensor records a doubling of brightness. This has a curious effect on image capture and it goes like this. Let's say my camera can capture 4,096 different shades or tones. Half of those 4,096 available tones go into representing just the brightest stop in my image.
That is half of the data I capture is being used to represent the very, very brightest things in the scene. Now half of what's left over from that goes into representing the next brightest stop. Half of what's left over from that goes to the next and so on and so forth. When all this plays out, by the time I get down to the darkest stop in my image, that is all the shadowy parts of the scene, I may only have 4 or 8 or maybe even just two tones available to represent all the data in those areas. Now, earlier you saw what happened when we used only a few tones to represent my image.
I got posterized. That same thing can happen to the shadow areas in your image, because of the nature of linear capture. Worse though is that because there's so little data for those areas, the signal-to-noise ratio of your shadow images degrades, and so you end up with shadows that have ugly noise patterns in them. Colored splotches usually. If I want more data in the shadow areas, then I can choose to underexpose. Then the brightest things in my image are not so many stops away from the shadow tones that I end up with too little data for the darkest areas.
Of course, then my bright things aren't very bright. My image looks dingy. In other words, if I want to expose normally, that is to ensure good highlight detail and have a good level of overall brightness, then there's a very good chance that I'm going to have noise in my shadows, simply because the camera can't capture enough data down into the shadow areas. With HDR techniques, I don't have to worry about this.
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