F-stop, controlled by the aperture ring, controls another part of photography called depth of field. A basic definition of depth of field is the distance from the nearest point in focus to the farthest point in focus. When you focus on a subject, there is an area in front of the subject that will be in focus and there is an area behind the subject that will be in focus.
The distances in front of and behind the subject that are in focus can be changed to increase or decrease this area. This is all controlled by the F-stop setting. When the F-stop is at a low number like F2.8, the area in focus in front of the subject and behind the subject will be very short, which is also called a shallow depth of field.
When you move up the scale of F-stops toward F22, however, the distance in front of and behind the subject that will be in focus will get bigger.
So, for example, let's say that you focus on a subject that is 10 feet away from the camera. When your camera is set at F2.8, everything about 1 foot in front of the subject will be in focus and everything about 1 1/2 feet behind the subject will be in focus. But, when you change the F-stop setting to F22, these ranges of focus will dramatically increase.While still focusing on the subject that is 10 ft away from the camera, at F22 everything that is 4 feet in front of the subject will be in focus and everything behind the subject will be in focus going almost to infinity. This is also known as a deep depth of field.
So, starting at F2.8, the depth of field is very shallow, and moving the F-stop towards F22 will make the depth of field change from shallow to medium to deep.
If you want to know how much will be in focus when you are about to take a photograph, all you have to do is look at the depth of field index ring. This ring is in front of your aperture ring and has a line in the center of it with a series of matching numbers on either side of the line ( usually 4, 8, 11, 16, 22 ). The line points to the distance of focus on the focusing ring and the numbers on each side of this line are the F-stop numbers. As you can see, 2.8 isn't on there. This is because the depth of field is so shallow, that it can't be represented on the index ring.
For an example, grab your camera and read on. ( This information is based on a 50mm lens as diagrammed in Camera Anatomy ) Set your F-stop to F22. Turn the focus ring until the line from the depth of field index is pointing right up at 10 feet. On the depth of field index ring, there will be a 22 on either side on the line.
Looking to the left side of the line you can see the 22 is lined up between 5 and 7 feet on the focusing ring, closer to 5 ( approximately 6 feet ). This is the beginning point of the field of focus. So everything starting 6 feet in front of the camera will be in focus.
Now, looking to the right of the line, you can see the 22 is lined up at the beginning of the infinity symbol. This means that almost everything to infinity will be in focus.
So there you have your depth of field. The range of focus starts at 6 feet in front of the camera and goes well beyond almost to infinity. Once you get the hang of it, it's really easy.
You might wonder why you need depth of field. Depth of field helps the audience of your photograph focus on what you think is important. Let's say that you want to focus on a face in a crowd, but you won't want the audience to be distracted with every other face. Using a shallow depth of field (F2.8) will do the trick! The face you want to give attention to will be in focus but everything else will be out of focus, thus centering all the attention on that face.
But, there may be times you want to give importance to the entire frame, like a photograph of the Grand Canyon. Using a deep depth of field (F11) will keep nearly everything in focus, thus giving importance to the whole scene rather than a specific part.
There may also be times when you want to use a medium depth of field (F5.6).