Today's cameras are marvelous things. As we press the shutter release, the shutter opens, just the right amount of light passes through the lens (a marvelous instrument in itself), and we get a good picture. If we're lucky.
The camera does many things well, but it only does what it's told. If we close our eyes, point the camera at the sky and press the shutter release, we'll get a picture of the sky. It will have clouds if clouds were in the sky, birds or airplanes if they were there, but exposure and composition will probably not be too good. In order to get good pictures, we will have to be in control of everything, including the automatic features of the camera.
For example, if our camera has automatic focusing, we'll have to make sure that the focusing eye is pointing at the subject. We can lock in the focus on the subject and then change the composition if we want to, but we must remember to do that. We must also know that our camera has the capability to let us do that (almost all of today's film and digital cameras have this feature).
We must also control exposure on an auto-exposure camera. If the subject is too bright or too dark, our pictures will be underexposed or overexposed. Scenery photography can result in some great pictures, but we also want them to be properly exposed. In a landscape shot with a lot of bright sky, the sky will determine the exposure. The result would be an underexposed foreground. For a properly exposed shot, tilt the camera down slightly to get the exposure reading, press down halfway on the shutter release to lock in the exposure, recompose the shot and take the picture.
Another common situation: On a bright afternoon you are taking a picture inside a house and your subject is standing in front of a window. Your meter will determine the exposure from the light coming in the window, not from the subject. The result will be extreme underexposure and the person's features will be very dark and obscure.
To get a more accurate exposure reading, turn away from your subject and point the camera at something with about the same brightness as the person, and lock in the exposure as described above. With most cameras, the substitute subject should be about the same distance from the camera as the primary subject because locking in the exposure also locks in the distance setting.
Keep in mind that the camera records everything that appears in the viewfinder. When we look at a scenic landscape or an exciting event, it's possible for our minds to block out everything but the most interesting aspects of what we are observing. While standing at the edge of the Grand Canyon, we become so engrossed in its grandeur that we will overlook the candy wrapper on the ground. At a sporting event, we don't see the hand waving in front of us. The camera isn't that selective. It will record everything in its view.
In our picture taking, we should discipline ourselves to look at everything in the viewfinder and decide whether or not we would want it in a picture. We can concentrate on the subject, but we should also decide how much of the other stuff we want. Would it make a better picture if we zoomed in? Zoomed out? Moved to one side or the other? Picked up the candy wrapper? The decisions we make before pressing the shutter release will have a lasting effect.
Okay, if you use a digital camera, you can look at the images on the LCD monitor on the back of the camera, and make adjustments in your shooting at that time. But wouldn't it be better to have gotten it right the first time?