Originally posted by marcusBMG This is normal. if you mount on the camera without a lens then the camera pushes the levers together.
If the lens aperture is indeed not being opened up correctly when mounted then a mechanical problem is a real possibility. With the aperture ring on "A", any misalignment or disconnect along the linkages will leave your lens on f22 = way dark pics! Inspect the aperture lever on the lens to see if it seems to be working correctly and isn't bent or tweaked out of alignment. ditto the tc. Inspect the operation of the aperture lever with tc mounted on the lens.
When you posted you did set mode on the camera to Av didn't you? cos if you didn't that's probably the problem! An electrical probelm could still be the cause: is the camera showing f numbers on the lcd display (in Av, aperture ring on "A")?
Do some more tests - try as well manual mode as well does that work?
Thanks to all for suggestions. I decided to start again with a proper tryout with the KAX teleplus.
Mounted my Pentax 20d on a tripod in my backyard and focused on a nearby gas heating flue in the house behind. 9.30am clear sky sun behind me and camera shaded by the verandah. Attached the KAX teleplus followed by Sigma 100-300mm 1:4.5-6.7 DL. Input focal length screen appeared set it to 600mm. ISO set to 800 camera mode set to Manual. Focused on gas flue with the lens set to f4.5 (light needed so I could see image), then switched the lens back to A. Turned the aperture wheel on the camera to as wide open as the dial would allow F1.4, the shutter speed indicated 200 this is with exposure bar in the centre, took photograph with a cable release cord attached to camera. Photo came out as under exposed. Turned shutter wheel to 60 left aperture on F1.4 photo came out near perfect.
As a further test I detached the KAX teleplus and attached the same Sigma lens to a TELEMORE 95 II 7•K•M•C teleconverter all manual no electronics at all to the Pentax 20d and got the same photo of the gas flue, Manual mode setting, same ISO 800. The same looking photo was taken at F4.5 Shutter speed 640. I placed the KAX back on the camera turned the camera aperture to wheel to F4.5. Shutter on camera indicated 20 took photo and also came out with the same type of exposed picture. Turned lens aperture dial back to F4.5 camera gave me F-- pressed green button shutter said 640 took photo and the same result as the TELEMORE. Conclusion, appears that Teleplus maybe working in a fashion or that it is very, very slow through the camera, for my needs virtually unusable for bird photography. Will have to start again looking for an auto teleplus or converter, unless some body has a better suggestion.