Today I took a 2xTC with PK/A mounts apart.
I took out all the glass so that I can use it as a 50mm tube with electrical contacts.
It works very well.
First I placed this new tube before ( closest to the camera mount) my Kiron 105mm f2.8 and the Tokina AT-X Macro Extender (TME) that I have been using.
That didnt quite work as well as I had hoped. I think that the TME is ment to be closest to the filmplane. Using a tube infront of it made the image soft and a bit strange.
I swapped the tube and the TME and voila. I got somewere around 3:1 with PK/A all the way.
Shooting with this was really hard. This shot is handheld with flash and not fully zoomed in but cropped and finally scaled down a lot. Also some PP to sharpen it up a bit.
Distance from tip of the lens to subject when fully zoomed in is about 6 cm.
Im not sure going 3:1 when shooting bugs like this is the best idea. When I zoom out fully I get slightly less then 1:1 at about 20cm distance but often I need even less.
I would prefer having a range from 1:2 to 3:1 but Im not sure thats possible