An extender optical system, for insertion between a photographic main lens and a camera body for changing the focal length of the lens system as a whole to the long focus side, is formed of, in order from the object side, a first lens group having negative refractive power, a second lens group formed of a single negative lens, and a third lens group having positive refractive power. The kernel is also able to inspect all inter-process communications originated by the real time threads running in such processes, in order to ascertain what other processes they invoke, and, if they have the potential to block a real time operation, the kernel is arranged to lock the areas of memory these processes reference. The terminal includes a recessed portions or holes for engaging the plate-like terminals with the ribs in a regular mounting position without gap left therebetween. A purge valve and pressure measurement means are installed in fluid communication with the valve box. The perturbed 3D model may be displayed.