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When we refer to dynamic range, we are speaking simply of a range of brightness. For example, your eyes have a dynamic range. That is a range of brightness in which you can see. If you go into a room that's so dark that you can't see anything, then you're outside the dynamic range of your eyes. Similarly, if you're driving your car west at sunset, you might be having trouble seeing the road. You might think of yourself as blinded by the sun. But another way of thinking about this is that you are again outside the dynamic range of your eyes. You are experiencing more light than they can handle.
Obviously, the more dynamic range you can perceive, the more places where you'll be able see. Dynamic range also impacts your perception of color, because colors have a brightness. So when you see a greater range of brightness, you can possibly perceive a greater range of color. You should already be familiar with the photographic concept of a stop of light. Every time the light in a scene doubles, we say that it has increased by one stop. Halve the light and we say that it has decreased by one stop. Therefore, stops are a measure of light.
And if you measure the dynamic range of your eye, that is, the range from the darkest light to brightest light that you can perceive, you find that your eyes have something in the range of 18 stops worth of dynamic range. Now, you can't see that entire range at once. If you walk out of a dark room into a bright light, it takes your eyes a bit before they can adjust to perceive that brighter part of their range, but they can't adjust themselves to see this tremendous range of 18 stops worth of light. By comparison, your camera probably has a dynamic range of just 10 to 12 stops.
In other words, your camera might have just barely better than half the dynamic range of your eye. Now this is a very important concept to understand whether you are interested in HDR or not, because you might see a scene in the world with very broad dynamic range, take a picture of it, and then be disappointed when you get home and find that there's no detail in the brightest areas or that shadows have gone completely black. For example, say you came across me right now standing in front of a bright background like the sky. To your eyes I'd probably look something like you're seeing me right now.
That is you would be able to see detail in the sky and on my face. Thanks to your eye's tremendous dynamic range, you'd have no trouble seeing this whole scene. Now say you wanted to take a picture. So you raise your camera and you shoot. Most likely you would end up with something like this. I am now dark and shadowy. Most auto features will expose to preserve details in the highlights, because blown out highlights are always less attractive than dark shadows, but in this case exposing for the highlights, the bright background, leaves my face too dark.
Again, the camera just doesn't have enough dynamic range. You might review the image and say, oh, that's no good, and so dial in some over-exposure using maybe your camera's exposure compensation control. When you do that, I'll brighten back up but the sky over exposes like this. So now you have got me, but no sky. In this case, there's nothing you can do in camera with a single shot to capture this scene the way your eye sees it. It is simply beyond the dynamic range of the camera. Now, obviously I'm not really standing in front of the sky like this.
I'm standing in front of a green screen, because our video cameras are just like you're still camera, they cannot capture a scene the way your eye does. They don't have the dynamic range. So we have concocted this simulation through some clever compositing on the part of lynda.com's excellent graphics team. In the real world, you will find a lot of situations like this one. Anytime you shoot someone in front of a window in your house for example, landscapes are often subject to dynamic range issues also, because you often have a very bright sky and a foreground that's darker than the camera's dynamic range can reach, if it is exposing properly for the sky.
Basically, anytime there's a big difference between the brightest and darkest objects in your scene, you have probably gone beyond the dynamic range capabilities of your camera. To be a good photographer, you have to understand what your camera is capable of seeing and know how that differs from what you are capable of seeing. If you can recognize when you're facing a scene where your eye is showing you more dynamic range than what your camera will be able to capture, then you can employ techniques to try to capture all that extra range.
We refer to those techniques as HDR or HDRI, High Dynamic Range Imaging.
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