Depth-of-field refers to the amount of the scene that appears to be sharp in your shot. Unlike our eyesight, which compensates for limited depth-of-field by quickly scanning different areas of the scene and focusing on near and distant points, camera lenses must obey some simple physical laws. Chiefly, depth-of-field extends in front of and behind the point you’ve focused the lens on. This zone of sharpness is not distributed evenly, however: it extends twice as far behind the focus point as it does in front of it.
The amount of depth-of-field in your shot is determined by three factors – the focal length of the lens, the aperture used and the distance from the subject.
1. Focal length. The shorter the focal length of the lens the greater the depth-of-field, and the longer the focal length, the smaller it will be. In general, using wide-angle lenses you’ll find that most of the scene is in sharp focus, while telephoto lenses are better for giving shallow depth-of-field, allowing you to throw distracting background elements out of focus, beyond depth-of-field.
2. Aperture. The wider the aperture you use the less depth-of-field you’ll get. So, using a wide aperture of f/4 or f/2.8 will mean that less of the scene will be in sharp focus than if you use a small aperture, such as f/16 or f/22.3. Distance. The closer you are to the subject the less depth-of-field you’ll have to work with. You’ll find it easier to get greater depth-of-field if you focus on subjects further away from the camera. Conversely, if you want shallow depth-of-field, move closer. This is most apparent when shooting close-ups, where you’ll get very shallow depth-of-field.