This parameter is linked by expression to the Houdini property, Focus Distance, in the sampling Tab.
Perspective cameras have a
flat_field_focus attribute which is set to true by default to match the standard thin lens camera model. This prevents over blurring away from the optical axis. Some renders might change, especially with wide FOV angles or very shallow DOF. Set
persp_camera.flat_field_focus false to get the previous behavior.