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Depth of field is the limitation of perceived sharpness within a photographic image. The greater the depth of field, the more of the image from front to back that appears sharp. An image with a shallow depth of field has a short and more specific depth of sharpness.

In photography, careful use of depth of field can be a very powerful tool indeed. It can force viewers to focus only upon that which is sharp, by utilizing a shallow depth of field. As our eyes are not comfortable in viewing unclear images, we then tend to look at the parts of an image that is sharp. Our gaze will then focus upon that part of the image. The other unsharp parts of the image are blurry and not worthy of our attention. This use of a shallow depth of field is particularly well suited to portraiture. As long as the eyes are sharp, we will disregard most other things if they aren't pin sharp. People and animals tend to look at the eyes first. So the eyes need to be sharp in most portraiture photography.

Landscape photography is generally at the opposite end of the scale of depth of field. The vast majority of landscape images need a very long depth of field. This is due to the fact that landscapes generally are trying emulate an actual scene as we see it. Viewers are usually drawn into the image by its great depth of field.

You can control depth of field in two ways. The most common way is aperture control. The smaller the aperture (the larger the number ie. F22), the greater the depth of field. The larger the aperture, (the smaller the number like F2.8), the shallower the depth of field. The apertures between have a depth of field proportionate to the aperture selected along the scale. 

The second means of controlling depth of field is by using a camera or lens that can tilt the lens forward or back. This lets the focusing plane of the lens to be more inclined to the plane of focus of the subject matter. This provides a much better depth of field without a change of aperture. It is one of the major reasons for using bellows type cameras, or tilt lenses. With such a camera or lens, one can have a huge level of control over depth of field at any aperture.

Depth of field is also dictated by the focal length of the lens, and the camera format using the lens. For instance, a wide angle lens always has a much greater depth of field than a telephoto lens. A very wide angled lens such as a 14mm lens has a depth of field so great that it almost doesn't need focusing. A 600mm telephoto lens has a shallow depth of field. Unless focused upon long distance subject matter, the depth of field will be very limited. 

On the other end of the scale are macro lenses, which are able to focus on very close objects. Once you start moving in for short focusing, the depth of field again becomes quite shallow. The closer you get to the subject, the less the depth of field becomes. In extreme close-ups the slightest movement will cause the image to go out of focus entirely.