Hello Peter,
Depth of field in arnold's perspective camera works as a sort of "ideal" thin-lens camera, derived as an extension of an "ideal" pinhole camera, where the size and relative distance between the lens elements and the film are abstracted away through direct control of the camera's angular field of view.
The cameras are "ideal" in the sense that in contrast to physical cameras where changing settings like the focal length of a lens or the aperture impose a balancing act in order to preserve other quantities like vignetting, exposure and field of view, arnold's built-in cameras were designed to allow independent editing of the different camera attributes.
Depth of field defocusing effects can be enabled by setting the "focus_distance" (in world units) and the "aperture" (radius, in world units) on cameras that support this feature. You can see see some examples in the MtoA documentation here: https://docs.arnoldrenderer.com/display/A5AFMUG/Cameras
