The ability of the camera lens to vary the amount of the scene that appears in focus, depth of field, is one of the most useful creative tools available to the photographer. Appropriate use of depth of field can often “make or break” a photograph.
Say for example you were taking a head and shoulders portrait of a friend in their back yard, unfortunately, their yard leaves a lot to be desired, in terms of attractive backgrounds for the portrait. But with the depth of field control of your lens you can over come these difficulties, by choosing a shallow depth of field, the resulting picture should result in only your friend appearing in focus with the unattractive background no more than an indistinct blur.
In other situations it may be desirable to have as much of the scene in focus as possible, such as photographing family members on holidays, so people who are looking at the photograph will be able to see clearly where you and your family were. So it is always important to consider the appropriate depth of field required before taking each photograph. Depth of Field Controls
There are three main factors that control the how much depth of field your lens produces in a given situation, these are the aperture setting on the lens, your distance from the subject and the focal length of the lens you are using.
Aperture is critical to depth of field. The aperture of the lens is usually controlled via the ring closest to the camera body. The aperture itself is an adjustable diaphragm, a series of overlapping metal blades that operate together much like the iris of the eye. The numbers on the ring refer to a particular aperture, the numbers themselves are actually fractions which refer to the size of the aperture relative to the size of the lens. Thus an aperture of f16 produces an aperture hole 1/16th the size of the lens. Apertures work in factors of 2 with each other, the apertures each side of your chosen aperture either admits twice as much or half as much as your chosen aperture, so f11 allows twice as much light into the camera as f16 and f22 allows only half as much light to reach the film as f16.The amount of light that a particular aperture admits is consistent no matter what lens is used with your camera, which makes life easier in situations where you have calculated your exposure and you then decide to change lenses. The smaller the aperture (or the larger the number on the lens barrel) the greater the apparent depth of field.
At f16, for example, most normal (50mm) lenses focused at a point 3 meters away will make the scene appear sharp from 1.5 meters to infinity. On the other hand if we were to choose an aperture of f2 under the same conditions, only the subject will appear sharp, both the foreground and background will appear blurred.
Subject distance also affects depth of field. In general, the closer your subject, the shallower the depth of field. Even at f16, if you focus on a subject 1 meter away with most normal lenses, the depth of field will be less than 30cm, at f2 if this was a portrait your subjects eyes may be in focus while the nose and ears are blurred. As you back away from a subject the depth of field increases rapidly.
The focal length of a lens plays a role in depth of field too. The shorter the focal length, the more depth of field you will get at a given aperture at the same subject to camera distance.
Thus a 28mm wide angle lens set at f11 produces greater depth of field than a normal lens set at the same aperture and the normal lens produce greater depth of field than a 200mm lens.