Here's Pook's example from the link I posted above.
The important thing to understand is that each of the above combinations of shutter speed and aperture will admit the same amount of light into the camera and result in a correctly exposed image. The smallest aperture, f22 will admit a tiny column of light for 1/8 second. The largest aperture, f2.8 will admit a larger column of light for a much shorter time - 1/1000th of a second. The point is the amount of light admitted to the camera will be the same. It will also be exactly the same amount of light for all the combinations of aperture and shutter speed in between those two examples. The shutter speed determines how long light will be admitted for, and the aperture determines the diameter of the column of light which will be admitted .
So which combination do you choose? You're thinking of it in the wrong terms Mike, because they'll ALL give the technically correct amount of light for proper exposure.
Well there's little point in choosing f22 at 1/8 of a second if you're trying to freeze a F1 car zooming past, and you'd never hand hold the camera for any shot at 1/8 of a second anyway, it's far too slow. You'd go for a combination which gave you fastest shutter speed to freeze the f1 car obviously - either 1000th or 1/500th of a second with the corresponding correct aperture.
On the other hand if you're shooting a landscape then you need a much smaller aperture if you want the clump of grass 5 feet in front of the camera to be in focus as well as the rolling hills in the distance. So you'd choose f22 or possibly f11. Here's the next problem - can you hand hold the camera at 1/8 or 1/15 of a second? Nope. not unless you're superhuman!
Solution? - go get a tripod if you're interested in landscapes.
If you wanted to photograph your wife or mistress (
) close up then you'd choose a large aperture to throw the background out of focus. so you'd have no trouble hand holding the camera at f2.8 and 1000th of a second.
That's the gist of it Mike - it depends very much on the subject.
At the stage you're at now the best thing you can do is use Aperture Priority Mode or Shutter Priority Mode - (Canon mark them AV and TV). If the aperture is most important (landscapes etc), then use AV where you simply set the aperture and leave the camera to choose the corresponding correct shutter speed.
If shutter speed is more important (action shots etc) then use TV mode where you choose a suitable shutter speed and the camera sets the aperture.
Hope that helps mate.
