What is aperture?
Shallow Depth of Field
The aperture of a lens is the diameter of the lens opening. The aperture that you set impacts the size of the opening. The larger the opening the more light that gets in. The smaller the opening the less light.
Aperture is measured in f/stops. For example, f/4, f/8, f22, etc. Moving from one f/stop to the next doubles or halves the size of the amount of opening in your lens (and the amount of light getting through). Keep in mind that a change in shutter speed from one stop to the next doubles or halves the amount of light that gets in also. This means that if you increase one and decrease the other you let the same amount of light in.
One thing that causes a lot of new photographers confusion is that large apertures (where lots of light gets through) are given smaller f/stop numbers and smaller apertures (where less light gets through) have larger f/stop numbers. So for example f/2.8 is in fact a much larger aperture than f/22 and will allow for more light to get through.
Depth of Field and Aperture
Depth of Field (DOF) is that amount of your shot that will be in focus. Large depth of field means that most of your image will be in focus whether it's close to your camera or far away. Small (or shallow) depth of field means that only part of the image will be in focus and the rest will be blurry.
Aperture has a big impact upon depth of field. Large aperture (smaller number) will decrease depth of field while small aperture (larger number) will give you larger depth of field.
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