Focus - Selective focus

By selectively focusing on a specific subject within a scene or selecting an aperture that produces limited depth of field (so that only that area is in focus), the viewer is predictably drawn to the area in focus.

This technique can act as a sign to the viewer, it announces, the area in sharp focus is the subject, this is important look at this, while the out of focus areas might be simply seen as the ground that the subject sits against and are generally considered of less importance. It can be used to create a psychological emphasis on those parts of the image in focus by pulling the audience towards them. Generally an audience expects object in the foreground to be sharp against an out of focus background. In terms of space, some contradiction can arise when the background is in focus while the foreground is out of focus. Often more concentration is needed to read images like this.

Detail from Mural 1 The last Rivers Song 1984 Lloyd Godman

In this image from the Last Rivers Song Series the rock embankment in the background is in focus while the wave flowing down the river is out of focus because it is against the face of the underwater housing. As it takes the audience sometime to read the image and understand the relationship of the rock to the out of focus shape of the wave, it creates a sense of visual mystery in the image.