The depth of field refers to which part of the image you want to appear in focus from foreground through to background and is determined by a combination of the aperture setting and the focal length of the lens.
It is concerned with making the light from different subject distances in the scene all come to focus at one lens setting.
In practical terms, we care about it because it is an extremely important element in the overall composition of your photographs . Therefore, it is very important to be able to control it and make it work for your pictures and not against them.
The adjustable size of the aperture in the lens has a great impact on how much of your image appears sharply focused and thus direct the attention of the viewer.
The widest aperture (smallest f-number) gives the least or shallow depth of field. These images generally have a single object sharply focused, with the rest of the image blurry. This is a great and easy way to direct the viewer's attention.
On the other hand, smallest aperture (highest f-number) results in greater range of items that appear clearly focused and is said to have a large depth of field.
Let's look at an example...
In both images below, the lens focus setting remained the same, but changing the aperture setting from f4.0 to f16 greatly increases the zone of sharpness in the image.
The image on the left that uses wide aperture at f4.0, results in very shallow depth of field - only where the lens was focused is really sharp.
The other image taken at the small aperture of f16 shows much greater zone of sharpness where almost everything is in focus.
As well as changing the zone of sharpness in an image, each aperture f-stop in the series also doubles,or halves, its brightness.
Take advantage of the fact that the digital camera, via its LCD screen, makes it easy to check the zone of sharpness straight away. This allows you to shoot a range of images at different aperture settings and, after reviewing the zone of focus result in each photograph, make a final aperture selection.
There are two other effects that we need to be aware of.
The first one is that the depth of field increases with the distance of your subject.
Pactically this means that for close ups there is less depth of field and you need to focus very precisely, while there is a greater depth of field when your subject is further away. Graphically this can be seen as follows:
Secondly, the longer the focal length (or the greater the magnification) of your lens the less zone of focus it gives, even with the same aperture and subject distance. Again, graphically we can see it as follows:
An example of such a lens is the telephoto lens , which tends to have shallow focus.
A wide-angle lens on the other hand, tends to create images with greater depth of field regardless of the aperture setting, which is why they are widely used in landscape photography. The greater zone of focus ensures that your picture contains the maximum information. As limiting the zone of focus in a landscape picture to the foreground would not make much sense.
As you can see learning to manipulate the depth of field to your advantage allows you to have creative control over your image . You can direct the viewer's attention to a part of the image, and "suggest" the surroundings without introducing too much clutter in the background to distract the viewer. Alternatively, you can choose to have greater zone of focus and show all the detail at several different distances when you feel it is relevant.