below was one of the first breakthroughs in my photography career. thanks to Matt for this from his site.
*************
Now, let's tie it together. For any given amount of light and film speed, there is one correct amount of light for a correct exposure but this amount of light can be achieved with a whole series of correct exposures. This is because both the shutter speeds and f/stops double and halve. If the camera gives you an exposure of 1/125th at f/8, you can get the same amount of light to the film by setting the camera to 1/250th at f/5.6 (twice the light for half the time) or 1/60th at f/11 (half the light for twice the time) or 1/30th at f/16 (one quarter the light for four times as long), and so on. In each case, the amount of light on the film is identical; what changes is your depth of field and your ability to stop motion. This gives you creative control over how your pictures will turn out.
