I've lifted this from a previous thread on exposure setting. Hope it helps
Correct exposure can always be obtained by using several different combinations of shutter speed, lens aperture and ISO. You can mix and match them to give different pictorial effects - like wide lens apertures (low f/number) for shallow depth of field, or fast shutter speeds to freeze action – but the actual exposure level is always the same for a given situation. You might find it easier to use Av or Tv or M (instead of Program) to get the pictorial effect you want, but if you understand what the camera is doing, it makes no difference to the actual exposure.
Exposure setting is simply about getting exactly the right amount of light on to the camera’s sensor. This will give a picture that is nicely balanced with bright highlights, rich shadows, clear mid-tones and colours.
Try this analogy, and compare exposure setting to filling a glass with water.
You need a certain amount of water to fill the glass. Same as you need a certain amount of light for the correct exposure.
The tap is your camera, and you can either turn the water on full (wide aperture, low f/number) or just a slow trickle (small aperture, high f/number).
And you can leave the tap on for a short time (fast shutter speed) or for as long as you like (slow shutter speed).
You can fill the glass either by turning the tap on full for a short time, or slowly for a long time. The glass still gets filled the same.
Changing the ISO changes the size of the glass. A low ISO number is a big glass needing a lot of water, and high ISO is a small glass, needing less.
At all times, the camera knows the size of the glass, it knows how much water comes out of the tap as you turn it, and it knows how long you can turn it on for.
The only variable the camera doesn't know is how bright the sun is shining - that's the water pressure in this analogy. Lots of light is high water pressure which will gush out of the tap fast, even if it's only turned on a little, but if it's darker then the water pressure is low and it will only trickle out even when the tap is full on.
In exposure metering, the camera measures the brightness of the light, the water pressure, and because it knows exactly what all the other factors are, it then presents you with a combination of camera settings - how hard to turn on the tap, and for how long - to fill the glass exactly, and give you correct exposure.
To continue this analogy a bit further, if we imagine a pint beer glass then filling to the line is correct exposure, below the line is under exposure, and above the line is over exposure.
The thing that confuses most people about exposure setting is the halving and doubling relationship between all the exposure controls. Once you’ve got the hang of this, the different exposure modes are easy to understand and adjust. For example, 1/125sec is obviously twice as long as 1/250sec, and ISO 100 is half as sensitive to light as ISO 200.
Less obvious is that f/4 passes twice as much light as f/5.6, which passes twice as much light as f/8, and so on. The key with f/numbers is that they are not linked by the number 2, but by the square root of 2, which is 1.4. Who knows why, and it’s been confusing photographers for generations, but now you know, the mists should be lifted!