What Is An F-Stop?
One of the most confusing things about photography for many people is the f-stop. However, its not nearly as mysterious or complicated as is often thought. To understand it, it helps to think about the process of making a photograph. In order for film to be exposed, creating an image which becomes a photograph, it must come into contact with light.
There are two things regulating the amount of light reaching the film: one is shutter speed, the other is aperture. Together these two determine what's called the exposure. The first thing to know about the f-stop is that it controls the aperture, thereby impacting the exposure. The second thing it does is control depth of field.
When you select an f-stop, you are telling the camera how much you want the diaphram to open when you take a picture. Put another way, you're telling the camera how large to make the opening in the lens that light passes through on its way to the film. The bigger the opening, the more light will hit the film. This is called the aperture.
Looking at the f numbers on a typical lens you'll see a series of numbers something like this:
1.7, 2.8, 4, 5.6, 8, 11, 16, 22
The numbers you see depend upon the lens being used (more on that in a future article). As the f number gets larger the aperture gets smaller. In other words the larger the f-stop number the smaller the opening in the lens will be when you take your photograph. As you move up the scale, each successively larger f-stop allows half as much light to enter the camera as the previous step. Thinking in reverse, each step lower you go on the f-stop scale allows twice as much light to enter the camera as the one just before it.
As you may have guessed from this discussion, one of the ways the f-stop is useful is in adapting to existing light - giving you the best possible image. For example, in low light conditions with no tripod you can (often) manipulate the f-stop until enough light is entering the lens to allow you to shoot at an acceptable shutter speed - one that is not too slow for successful hand-held operation of the camera.
Depth Of Field
The second function of the f-stop is to control depth of field. When looking at a photograph you'll notice that sometimes not all areas of the picture are equally in focus. Parts of the background may be more out of focus than those in the foreground or vice versa. The extent of "blurriness" in other parts of the picture as compared to the main subject is the depth of field. The clearer the entire picture the more depth of field the picture has and the softer or more out of focus things other than the subject are the less it has.
If you want a picture where the subject really 'pops' and everything else fades into the background, you want a larger opening in the diaphram meaning a smaller f-stop number. If you want a picture where everything in the shot is more or less in focus, you want a smaller opening in the diaphram meaning a larger f-stop number.
To see the effects of this and for help learning what f-stops produce what results, do a photo study of a flower or some other object. Take multiple shots using several different f-stops (be sure to record which shot used which f-stop). Then when you get the pictures developed, you'll have a visual to help you know which f-stop you should use for which desired effect - for the lens you were using at the time.
There is more to say on the subject of f-stop - but this should give you enough information to begin using f-stop settings more effectively. Once you learn to do that well you'll be surprised how the quality of your pictures improves!