TUTORIALS: DEPTH OF FIELD | Depth of field refers to the range of distance that appears acceptably sharp. It varies depending on camera type, aperture and focusing distance, although print size and viewing distance can also influence our perception of depth of field. This tutorial is designed to give a better intuitive and technical understanding for photography, and provides a depth of field calculator to show how it varies with your camera settings.
Big Ben and houses of parliament Depth of Field being used creatively - I feel this gives a much better effect than having shot the image with a smaller aperture which would give a bigger focus in the image (so Big Ben in the background would have been in focus too).
Depth of Field: The pictures above change the aperture setting of the camera lenses, creating a sense of depth in their images. The larger the apertures (smaller F-stop number) and closer the focusing distances, the shallower depth of field is. Other example of Shallow Depth of Field: http://digital-photography-school.com/17-beautiful-images-with-shallow-depth-of-field