The aperture is like the eye of the lens. If you think of it in those terms the concept is easier to understand. You know how your iris opens and closes in relation to the light. When it's dark it gets bigger to let in more light and when it's sunny it closes up so that you don't go blind? Well the aperture is just like the iris of your eye. You control light in 3 ways.... film speed, shutter speed, or aperture. All 3 work together, but aperture not only controls light but the dof. The f/ numbers in regards to aperture stand for the size of the opening. The larger the number (32, 22, etc.) the smaller the opening size and less light will come through the lens. The smaller numbers ( 1.8, 2, 4 etc.) mean the opening is bigger and lots of light will get in. When speaking in terms of aperture and depth of field together the thing to keep in mind is what Simon mentioned. The focal length of your lens will determine to some degree how the dof works, but the aperture opening is probably the most important. Larger openings (remember smaller numbers) will create a very shallow dof, while smaller openings (the higher numbers) will create a very sharp dof, more or most of the background will be in focus. Another thing to keep in mind is that dof and aperture sizes affect not just the background but the foreground as well. For example using a 105 mm macro lens like mine with an aperture setting of f/2.8 at it's closest focusing distance will create an image with almost everything both foreground and background out of focus, this is due to the longer focal length of the lens, the closeness of the subject, and the aperture being opened all the way. Not to confuse you more but f/stop simply put is the doubling or halving of a numerical value be it, aperture openings, film speeds or shutter speeds. Everything in photography is quantified in f/stops. The image above..... shot on 105mm Macro, at f/2.8, closest focusing distance. Everything is blurred except for a very narrow point of focus. Hope that helped and didn't confuse more. :~)
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