Differential focus

Differential focus is a camera technique where the photographer arranges camera and elements so that the primary interest is in focus, while secondary or competing interest is out of focus.

Differential focus is accomplished by the following:

Choice of a wide aperture, such as f1.4, f1.8, f2.8, which limits Depth of Field (DOF)
Use of a long focal length lens, such as 100mm or above, which also limits Depth of Field (DOF)
Where available, choice of FX over DX, since FX has a shallower depth of focus
Increasing the distance between the two objects
Getting closer to the nearer object, since depth of focus is proportional to distance
Awareness of hyperfocal distance
Previewing the shot by stopping down