Depth-of-field (DOF) is one of those things that can confuse a lot of new camera users. Yet, it is very important!
Choosing the right combination of apertures and shutter speeds is initially hard to comprehend. This article is written to help you understand those relationships to help you control your image's look, while maintaining correct exposure.
I'm going to attempt to explain these concepts with pictures.
Lets say you are taking a picture of a friend, who is standing 2 meters (~6.6 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 — each about 2 meters apart — with your friend in front.
You are shooting with a 50mm lens. You focus on your red-shirted friend's face, and take a picture. It looks like this:
50mm lens, Aperture: f/1.8, Shutter Speed: 1/6000th of a second
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." The 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 therefore only about 1/2 meter deep.
The f/1.8 is an "aperture" number. An aperture is simply an opening in the front of your lens controlled by blades. If you divide the focal lenght of the lens into its aperture f-number you get the diameter size of the effective aperure in the lens. In most cameras you should see the f/number somewhere in your viewfinder display but you won't actually see the effects of your aperture setting. This is because your auto single lens reflex (SLR) camera with an auto lens 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 a typical zoom lens start at about f/3.5 (big aperture), and go to f/22 (small aperture). The bigger the actual size of 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 I used for our example photos is definitely considered fast!
So what would happen if we closed the aperture down (also referred to as stopped down") to a medium-small 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 still 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 point).
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), or 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. The smaller f/22 aperture makes it easier to get sharp focus. Remember, you focused on the front girl's face in all these pictures. At first only she was in focus (f/1.8), and as the aperture got smaller more and more of her 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.
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.
50mm f/1.8 non-D AF Nikkor
The aperture opening is smaller at f/8 than at f/1.8 and less light is getting in through a 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.
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 Speed => Duration 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.
Keep on capturing time...
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