There are three main factors within your camera that affect the proper exposure of your photographs. They are the aperture (how much light you let pass through the lens at any given moment), the shutter speed (how long you let light pass through your lens), and the ISO (the sensitivity to light of your film or digital sensor). If you are set to the proper exposure and decide to change one of them, to then keep your exposure correct, means you have to change one of the others (or even both) to compensate. Each one, when changed, affects a different aspect of the final image in unique yet important ways.
Aperture, or f-stop, is not only how much light you are allowing into your camera at a given instant, it also directly affects your depth of field. A smaller aperture number (i.e. 2.8 versus 22) means more light is reaching the cameras exposure element (either film or digital sensor) and that there is a shallower depth of field. A higher aperture number (16 versus 4.0) means that your aperture is stopped down and less light is reaching your exposure element. However, you will have a much deeper depth of field.
Around this time, if you are newer to cameras, or at least to the manual settings of cameras, you might be starting to wonder what it is I mean by “depth of field.” Depth of field could almost be better stated if it was called “distance or depth of focus,” because it refers to how well objects at different distances from the camera are focused in the same photograph.
At smaller aperture numbers, such as f/1.4, f/2.8, f/4.0, or f/5.6 objects are in focus only if they are very close to each other or in the same plane. Sometimes, if you are very close to two objects, one of them could be in focus and the other one not, even if one was only an inch farther away. Crank down your aperture to one of the bigger numbers like f/22, and an object twenty feet away could be just as well focused as one fifty feet away in the same image. (This will also partly depend on lens length, but that will be a different lesson.) In short, as the aperture changes to allow more or less light through the lens, it also affects how much of what the camera is seeing is actually in focus.
As the aperture changes, the amount of light reaching your film or sensor changes. If you stop down the aperture to f/22, you are cutting down the amount of light. To compensate, you need to either increase your exposure time or increase the sensitivity of your film or sensor. For example, say you have a proper exposure when your aperture is set to f/8, shutter speed is 1/250 of a second, and ISO is 400. If you then adjust your camera by one full f-stop (aperture setting) to f/11, you would then need to adjust either your shutter speed (one full stop to 1/125) or your ISO by one setting (to ISO 800) to compensate your loss of light and maintain the proper exposure. Many cameras change their settings by half or 1/3 stops, but the numbers I am giving are for full. Adjust your settings accordingly.
If you have any questions or feel a point needs clarifying, feel free to ask questions. Next in the series will be a more detailed explanation of shutter speed.