Depth-of-field is one of those things that confuse a lot of new camera users. Yet, it is very important! Also, how the aperture and shutter speed works is initially hard to comprehend. This article is written to help you understand those relationships so that you can control your image's look, and correct exposure.
I'm going to attempt to explain these concepts with pictures.
Let's say you are taking a picture of a friend, who is standing 2 meters (~5.5 feet) away from you. About 2 meters behind your friend is another person. There is also a third person standing about 2 meters behind the second person. Three people total, about 2 meters apart, with the friend in front.
You are shooting with a 50mm lens. You focus on your friends face, and take a picture. It looks like this:
(50mm lens, Aperture: f/1.8, Shutter Speed: 1/6000th of a second)
F/1.8 is an "aperture" number. An aperture is simply an opening in the front of your lens controlled by blades. You can't see the aperture when you look in the front of your lens, normally, since your SLR camera allows you to focus with the aperture blades wide open, and out of the way. The aperture closes down to its selected setting when you press the shutter release to take your picture.
Apertures on your zoom lens probably start at about f/3.5 (big aperture), and stop at f/22 (small aperture). The bigger the aperture can get (the larger the opening) the "faster" the lens is considered. When you hear about a "fast" lens, someone is talking about a lens with a big maximum aperture opening. The 50mm f/1.8 lens is a fast lens. F/1.8 is fast!
Notice in the picture above that your friend (in red) is in good focus. The girl standing behind her, to the right, is not in focus, nor is the young lad even farther away to the left. This is the result of shooting with a big "aperture." F/1.8 is a big opening in the front of your lens. It also causes the depth-of-field, or "zone of sharp focus" to be shallow. Only the girl in front is in focus at f/1.8. Not much else is in focus, so there is very little depth-of-field. The depth-of-field in this picture is well less than one meter. Probably more like 1/2 meter. (~1.5 feet) The zone of sharp focus is only about 1/2 meter deep.
So what would happen if we closed the aperture down ("stopped" down) to a medium aperture like f/8? The picture below shows what that will do to the depth-of-field:
(50mm lens, Aperture: f/8, Shutter Speed: 1/500th of a second)
Notice how the girl in front still looks sharp, and the girl to the right is now in focus too. You focused your camera on the girl in front, but now the girl to the right is sharp too even though you did not change your focus control. The depth-of-field, or zone of sharp focus now extends past the girl in front and covers the girl in back. But, also notice that the boy to the left is still not in focus. The background is not in focus either. This image is the result of a medium aperture opening (f/8), not fast (f/1.8), not slow (f/22). Now, let's consider what happens if we "stop down" or close the aperture to f/22:
(50mm lens, Aperture: f/22, Shutter Speed: 1/40th of a second)
Aha! Now everything in the picture is sharp. An aperture as slow and small as f/22 makes it easy to get sharp focus. Remember, you focused on the girls face in all these pictures. At first only the front girl was in focus (f/1.8), and as the aperture got smaller more and more of the surroundings came into sharp focus (f/8 and f/22).
So, Depth-of-Field is simply the zone of sharp focus. It extends in front of and behind your focused subject, and gets deeper in both directions as you “stop down” your lens. If you set your camera to A mode, or Aperture Priority, you can adjust this powerful functionality to control what is in focus in your pictures.
Shutter Speed and Aperture Relationship
Notice also that the shutter speed changed as you stopped down your lens. At f/1.8 you needed 1/6000th of a second to keep the light from overexposing your image. A large, fast aperture lets in a LOT of light, so you can only let it in for a short time -- by using a fast shutter speed.
As you stopped down to f/8, your shutter speed moved to 1/500th of a second. The aperture opening is smaller at f/8 than at f/1.8 and less light is getting in through the smaller opening, so the light needs to come into the camera for a longer period of time. 1/500th of a second is a much longer time than 1/6000th of a second.
Then, notice how your shutter speed dropped to 1/40th of a second when you stopped down to f/22. At f/22 very little light is coming into the camera, so you have a long shutter speed at 1/40th of a second to let enough light in over a longer period of time. (1/40th vs. 1/6000th)
Conclusion
As you make the aperture opening smaller (f/22), you must let the light come in longer. As you make the aperture opening larger (f/1.8) you must let the light in for much less time. Does that make sense?
Aperture = Quantity of Light
Shutter = Time of Light
These two things work together to help you control the exposure and look of your image. With a fast aperture (large opening, f/1.8) you have very little depth of field, so you can isolate your subject from her surroundings. With a slow aperture (small opening, f/22) nearly everything in the image is in focus.
Experiment with your camera in M (Manual) or A (Aperture Priority) modes and learn how these relationships affect depth-of-field and the subsequent image's appearance.
http://www.cleanimages.com/Article-UnderstandingDOFandShutterAperture.asp
