Depth of Field (DOF) is probably one of the most difficult things for a beginner photographer to understand, probably because there are so many factors that affect it. Put simply, DOF describes the distance between the nearest and farthest elements in a photograph that appear acceptably sharp. This can be controlled by manipulating a number of different variables: The focal length, the aperture or "F-number," the size of the sensor and the distance from the camera to the subject. When used correctly, it can create an image that really stands out from the crowd.
The aperture acts in a similar way to the iris in the eye, controlling the amount of light that is allowed to reach the sensor. It is represented by a series of numbers called f-stops. Depending on what lens you are using, you could be operating at f-stops ranging from f1.2 all the way up to f32. In reverse of what you would assume, the smaller the number, the wider the aperture and vice versa. At wide apertures, more light is let in and the DOF is shallow. These settings create images that have a beautifully sharp subject, but a blurred out background. A smaller aperture makes a deeper DOF, bringing more of the picture into focus.
To practice this, set your camera at Aperture Priority (which might be represented by Av or A on your camera). On this setting, you can change the aperture manually and the camera will pick an appropriate shutter speed to expose the image correctly. Remember, the lower the f-number, the shallower the DOF. Try it out by taking your camera down to the lowest aperture it can manage, and focusing on an object. Telephoto lenses will have a more sensitive DOF than a wide angle lens, so try to zoom in to heighten the effect. Many cameras will have a DOF preview button that shows how the DOF will look, however if yours doesn't, don't worry, you can just take the picture and view it on preview screen. Now experiment by changing the aperture and comparing the images.
Where would you want to use a large DOF: When you are taking pictures of landscapes, where you want everything in the picture to be in focus. This is also important when taking group photos, where you want to get everybody's face in focus.
Where would you want to use low DOF: When you want to place emphasis on one part of the photo and remove any distractions. An example of this would be in portrait photography.
1. Foreground objects can help to elevate the sense of depth in the photograph, while moving away from a subject decreases this effect.
2. Wide angle lenses will have less noticeable effects on Depth of Field than longer telephoto lenses.
3. Try using ND filters to allow you to lower the f-number in brighter conditions (this is especially useful when shooting video)
4. Generally, prime lenses will give you the ability to shoot at lower f-numbers than equivalent similarly priced zoom lenses.
5. Reduce camera shake at smaller apertures by using a tripod to steady the camera. Alternatively, use flashes or a higher ISO to increase shutter speed.