The Aperture Setting
Aperture sets how wide the lens opening is when you take a picture. The wider it opens, the more light it lets in. Also, the wider it opens up the more blurry the background and foreground of your image. When you are setting the number on your camera, the lower the number, the wider it opens. I just try to remember that aperture is the one that is backward. The lower the number, the more light in and the more blurred the background is. Does that make sense?
Aperture Setting Exercise
Grab your camera and figure out how to set it to aperture priority mode. (on the canon rebel, you set it to AV, and you can raise or lower the aperture by turning the dial on the top left of your camera to the right or left.
Incidentally, in order to go to the lowest available aperture on your lens, you need to have it zoomed all the way out. I think most lenses that come with the canon rebel these days have a 3.5 lens, which means the lowest available aperture to you is 3.5, in order to dial it down that far, your lens must be zoomed all the way out.) It’s a good thing to keep in mind when buying a lens because you may want to be zooming in a lot, but the fact is you may have to be used to just getting closer to things rather than zooming if you need a wide aperture.
Find two objects, place one in the foreground, the other in the back and slightly to the side so it is visible. (make sure that wherever you are shooting has plenty of light!) Adjust your aperture so that it is at the lowest possible number. Focusing on the object in the foreground, take the picture. Now, raise up your aperture. If you started at 3.5, go up to 4.5. Again focusing on the object in the foreground, take a second picture. Raise your aperture up a third time, this time to maybe 5.6 and take a third picture. Now you can compare them to each other. The background object on the first photo should be the most blurry, and slowly become sharper in the subsequent photos.
With each adjustment to the aperture setting, the shutter opens less and less wide, letting less and less light in, which changes the depth of field, or how blurry the background is. In aperture priority mode the camera will choose a slower shutter speed to compensate for lack of light. If the camera allows you to see the information on each shot, make note of the shutter speeds and how they change in comparison with how the aperture changes.