I've left this problem alone for a few days tio think about it. Today I picked up the camera, fitted the lens, and everything worked! I have no explanation for this; I have done nothing to it and changed nothing. Perhaps it was just sulking?

Let's hope that this is not just an aberation and that it continues to behave itself.

Anyway, thanks to everyone for their help and suggestions. It is reassuring that there are good people out there willing to offer their time, expertise and experience.

The teleconverter has no electrical contacts just a completely smooth metal service, an ideal conductor between contacts unfortunately. It still works with other lenses.

In manual mode exactly the same thing happens, ie nothing works.

There are no switches on the lens body.

I am now thinking that when the lens was connected to the converter and mounted on the camera body, there might easily have been a short circuit between the various contacts which triggered some mishap in the lens' elecronics when the camera was swicthed on. If that is the case then a repair sounds as though it would be quite pricey and maybe not worth it. It would be a shame however to write off the lens.

I've just noticed that when the 28-300 lens is attached the correct information isn't shown on the screen relative to the selection made on the picture mode dial (is that the correct terminology?) For just about every position it shows "Av aperture priority."

Thanks Michael. I have wiped the contacts with switch cleaner on both camera and lens. Some dirt came off but I'm afraid it made no difference.

The contacts on the camera are spring loaded, and all work. Those on the lens are fixed.

I have two identical bodies. I have tried the 28-300 on both and get the same result, ie it doesn't work. I tried switching to live view on each body and got the message "Live view cannot be started". To all intents and purposes the lens seems to be "dead" as far as the bodies are concerned.

I've cleaned the contacts but it makes no difference. The aperture lever is working OK and does not appear to have been displaced at all.

I have a Sigma 10-20, which works OK on both cameras, and as far as I can tell the contacts and aperture lever look identical on both.

I might add that the camera works fine with another lens.

This lens has worked perfectly with the camera for several years without bother. I tried using it with a 2x teleconverter (non-electic connections) and the lens simply will now not communicate with the camera any more. I suspect possible linkage problems. Any ideas to try to rectify please?