Originally Posted by
Scotty72
Rick is not quite right but, close.
You are getting extension tubes (for super close up) and telephoto converters mixed up.
Extension tubes are for allowing you to get your lens within centimeters of the subject.
They often do have electronics in them but they are for focusing- nothing to do with metering.
The telephoto converters make your long lens even longer (eg a 200mm lens with a 2x TC) will give you an effective 400mm reach. But yes, you do lose stops of light with the TCs.
Scotty