Let's recap a few things here. We've been covering a lot of different modes. Program mode where the camera automatically picks a shutter speed and aperture for you, that will give you a good overall level of illumination. We've also looked at priority modes: shutter priority where you get to pick a shutter speed and the camera picks a corresponding aperture that will yield a good level of overall illumination; aperture priority mode where you get do pick an aperture, and the camera picks a corresponding shutter speed that will yield a good level of overall illumination. Are you sensing a pattern here? I keep saying that one phrase over and over over: "a good level of illumination." With all of the controls we've been looking at so far, the camera will always try to get you the same level of brightness.

I'm going to switch to aperture priority mode here real quick. I am metering, and I have dialed in an aperture of f11. If I change that aperture--I'm going to go down to f 5.6 here-- as I change that, the overall brightness in the scene has not changed. The camera is constantly picking a corresponding shutter speed that will preserve my overall light level. In this way, the camera is protecting me from ever over- or underexposing.

Now, believe it or not, as much as we've been talking about how you need to be careful about not overexposing your highlights, and that kind of thing, believe it or not, there are times when you want to over- and underexpose. We're going to spend the rest of this chapter talking about when those times are. Sometimes you will do it to ensure that black-and-white tones look correct. Sometimes you'll do it to improve color. There are lot of reasons why you might want to over- or underexpose. The key to making that happen is your camera's exposure compensation control. Now, most cameras use the same kind of readout for exposure compensation: this type of dial right down here.

In the middle is just a flat exposure, what my camera has decided is correct. And here I've got a little Plus sign and here I've got a Minus sign. On this particular camera, there is just a wheel on the back that I turn. For exposure compensation to work, you first have to meter. So I'm going to half-press to meter, and I let it focus. I'm going to lock my camera now. I'm going to switch my focus on my lens over to manual focus. Now, that it's focused, I don't ever need to re-autofocus on that. So that will stay there while we do the rest of this work.

Anyway, I half-pressed the meter, and now if I dial in a one stop overexposure, this little thing moves over here to 1--this is the Plus side. So I am seeing one stop of overexposure. The image is much brighter now. Let's go back, and you can see that again. Here is how the camera wants to meter. Here is one stop overexposed. So it's brighter now. I have actually managed to overexpose this shot. Let's go the other direction. I am going to go one stop under. My image is getting darker. Here is one and a third stop.

That's what this first little tick mark is. Here is one and two-thirds stop. Here is two stops under. So my image is getting progressively darker. So this is a very easy way to make an under- or overexposure. We'll talk about what those are for later. I just want to go over the control right now. Notice that as I am moving this dial around, the only thing that's changing is shutter speed. That's because I am in aperture priority mode. When you're in a priority mode, that parameter takes priority. I am in aperture priority mode, and I've dialed in 5.6, which means the camera will not change that.

If I tell it to underexpose, the only way I can do that is by changing shutter speed. Let's go in the other direction. I'm going to switch to shutter speed and dial in a shutter speed of say a 60th of a second. As I meter, it comes out at f 5. If I tell it to overexpose, that means it's going to want to open up the aperture. So as I go here, my number should go down, which it did. It went down to f 4. If I go to underexpose, my aperture should get smaller, because it can't change shutter speed, because I am in shutter priority mode, and then we're at 7.1.

So this is exposure compensation. In the next few lessons, we're going to take a look at what it's for.