I have a K-x and wanted to try shooting macro pictures. So I bought a set of Fotodiox extension tubes through the Fotodiox eBay store. The pictures I took came out fine and the tubes seemed to be working great. Until .... spots started appearing on my pictures.
As near as I can tell, the paint on the inside of the extension tubes flakes off and eventually gets on the sensor. Most spots came off the sensor with the use of a Rocket blower. I had one spot that wouldn't go off from a blower and then tried the Pentax recommended Image Sensor Cleaning Kit, O-ICK1 with the sticky ended stick & paper. After repeatedly using that I reached a point where it would remove no more of the spot. But the spot remained! I then had to get swabs and a wet solution to remove the remainder of the spot.
After this first started I found that if I assembled the extension tubes over a clean sheet of paper I could see black specks fall onto the paper. I have since soaked the tubes in soap for a day and then scrubbed them vigorously. I still get a few spots when I screw the tubes together but nowhere near as many as I originally had. Nevertheless, I am not sure whether these tubes will ever get mounted on my camera again. I just wanted to share my experience so others can be careful with these tubes.
On the plus side, I now have had an education in sensor cleaning and know how to do it if/when required.