There are two factors that are instrumental to how your pictures turn out. Shutterspeed and aperture. Together these two determine the exposure of the film (how much light that hits the film). On this page I will explain these terms and the relationship between them.


Shutterspeed

The shutterspeed is the amount of time the shutter of the camera is open. Basically the shutterspeed has two functions:
1)To allow you to take pictures in varying intensities of light
2) To freeze or blur action

The basic shutterspeeds are:

>>>> One step halves the light that hits the film >>
  1  ,  2  ,  4  ,  8  ,  16  ,  30  ,  60  ,  125  ,  250  ,  500  ,  1000  ,  2000  ,  4000
                                                << One step doubles the light that hits the film <<<<

These are all fractions of seconds, so 1 = 1/1 of a second and 250 = 1/250 of a second. Your camera may very well have numbers in between these, but the above numbers are the basic ones because each step to the left doubles the amount of light the film is exposed to and each step to the right halves the amount of light the film is exposed to. This light-ratio of 1:2 is called a full-stop and will become important when we discuss how to combine shutterspeeds and apertures further ahead.
As I mentioned earlier the two factors that you control with the shutterspeed are how much light reaches the film and to which degree you freeze or blur the action.
Examples: A slower shutterspeed will increase the light that reaches the film, thus allowing you to take pictures in less light, but it will also give the subject more time to move, thus possibly blurring the picture. A faster shutterspeed on the other hand will decrease the light that reaches the picture, thus making it hard to shoot in low light, but it will freeze the action in the picture.


Aperture

The aperture is the diameter of the opening in the lens. The aperture is formed by a diaphragm of overlapping leaves connected to an external ring and is changed with the controls on the camera. Aperture is denoted in f/stops. You get the f/stop by dividing the focal-length of the lens by the aperture. Thus if the focal-length of your lens is 100mm and the aperture right now is 25mm you would get the f/stop of 4 (100/25=4). A rule of thumb is: The larger the f/stop is, the smaller the aperture is and the less light hits the film.
The aperture has two functions:
1) To allow you to take pictures in varying intensities of light
2) To control the depth of field (how much of the picture that will be sharp)

The basic f/stops are:

>>> One step halves the light that hits the film >>
  1  ,  1.4  ,  2  ,  2.8  ,  4  ,  5.6  ,  8  ,  11  ,  16  ,  22  ,  32
              << One step doubles the light that hits the film<<<

As with shutterspeeds you may have numbers in between these on your camera, but these are the basic ones, because one step to the left doubles the light the film is exposed to and one step to the right halves the light the film is exposed to. As you can see this is the same light-ratio of 1:2, called a full-stop, that applies to shutterspeeds.
I mentioned above that the aperture has two functions, to control the amount of light that reaches the film and to control depth of field.
Examples:A smaller f/stop (larger aperture) will increase the amount of light that hits the film, but it will decrease the depth of field. On the other hand a larger f/stop (smaller aperture) will decrease the amount of light that hits the film, but it will allow you to have a greater depth of field, thus making more in the the picture sharp.
A rule of thumb here is: The greater the f/stop, the more depth of field.


Combining shutterspeed and aperture

The ability to combine different shutterspeeds and apertures is instrumental in taking good pictures. It allows you to create different effects in your pictures without risking over- or under-exposure. As I've mentioned above, the light-ratio between the adjacent f/stops and apertures is 1:2, a so called full-stop. As both shutterspeeds and apertures have this ratio it allows you to get the same exposure by increasing one of them one step and decreasing the other one step. The reason that you want to be able to do this is that in this way you can alter the depth of field and the sharpness/blur, of the picture, without under/over-exposing it even if the lighting condition doesn't change. This enables you to be creative in your photographing.
Examples: If you have a light reading of 1/500 at f/8 and you want slightly more depth of field without underexposing your picture you just increase the f/stop to f/11 and decrease the shutterspeed to 1/250 (one step in each direction). Now you have slightly more depth of field in the picture and still expose the film to the exact same amount of light. The same rules applies if you want to decrease the depth of field only now you go the other way to f/5.6 at 1/1000.

All this said, the best way to really understand how shutterspeeds and f/stops work together is to get out there and experiment on your own.

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