From the reading I have done on extension tubes, I understand that some are just 'dumb' tubes while others have electronics that may allow for metering and autofocus to still work. Further, from this ...
My extension tube set (Soligor for Pentax AF + P/KA) was not working correctly on my DSLR, so I checked the tubes, and found an interesting difference. It has three tubes (12mm, 20mm, 36mm). This is ...