>Can some one tell me the advantage if any for using a Camera >that the focusing motor is in the Camera rather than the Lens >itself. Other it makes the camera heaver and more xpensive and >the lens lighter and less expensive? > >and if a Camera such as a 7000 can use both how does it know >what motor to use when?
There is no advantage really but there are a wealth of older Screwdriven Nikkors out there, literally millions of them! There are some real optical Gems too! Some can be picked up for a song and all will function perfectly with a Camera with a Motor in the body but you will lose AF function on one without..
The contacts on the lens throat connect with the corresponding one on the lens mount and they communicate, simples... Basically just enough info to ID the lens and it's parameters to give the Camera body enough info to know what to do. Or enough rope to hang itself if you don't clean the contacts on a regular basis