I wanted to try for some back lighting. I still shot in Av mode (I think because I am lazy, not that I can't do manual.) However, with back lighting, you have to take charge and let camera know you're smarter than it is. I had my exposure compensation set to (+1/3) because the sand is lighter than the 18% gray the camera is looking for. So I've told the camera this scene is a little light, but there's not a way to tell it that it's back lit.
Enter the magic of exposure lock.
If you leave it to auto exposure, this is what you end up with. With all the light and dark spots, your average is still 18% gray, but nothing is right.
On my canon, the exposure lock is * button, next to the auto focus point selection button, and doubles as the (-) button during photo playback. When you use it, you are telling the camera to meter according to what you have in your viewfinder now, not what you have in your view finder when you press the shutter button.
So for this scene, I put a scene of sand and some bushes in the view finder, intentionally not using any of the overly bright sky, then pressed the * button. Then is moved my camera and composed the scene how I wanted. The result was much more like the scene my eye was seeing than the automatic exposure is, and I will take a totally overexposed sky over the automatic dark mess any day.
So, in short, when you push the * button, the camera calculates the exposure and sets the ISO, Aperture, and shutter speed, and doesn't change even if you recompose the scene. While exposure is locked, you see * in your view finder.