Normal viewing transforms act like a perfect pinhole camera; everything visible is in focus, regardless of how close or how far the objects are from the viewer. To increase realism, a scene can be rendered to vary sharpness as a function of viewer distance, more accurately simulating a camera with a finite depth of field.
Depth-of-field and stereo viewing are similar. In both cases, there is more than one viewpoint, with all view directions converging at a fixed distance along the direction of view. When computing depth of field transforms, however, we only use shear instead of rotation, and sample a number of viewpoints, not just two, along an axis perpendicular to the view direction. The resulting images are blended together.
This process creates images where the objects in front of and behind the fusion distance shift position as a function of viewpoint. In the blended image, these objects appear blurry. The closer an object is to the fusion distance, the less it shifts, and the sharper it appears.
The field of view can be expanded by increasing the ratio between the viewpoint shift and fusion distance. This way objects have to be farther from the fusion distance to shift significantly.
For details on rendering scenes featuring a limited field of view see Section 14.3.