There ought to be enough accumulative photographic knowledge amongst members of this esteemed forum to solve my problem. Well, hare's hoping anyway. Problem...To fit a Canon FD lens onto a bellows that has a Pentax M42 x 1 female thread. The manual aperture ring on the lens must function but data is not fed to the M42 thread. The obvious answer is to use a FD to M42 adaptor.....very difficult. According to the Pentax forum such an adaptor has never been commercially produced, if my memory serves, it's to do with the back-focus or some such. http://photo.net/canon-fd-camera-forum/00XDcp A few special adaptors may have been made to order by someone like Novaflex. I have done it on paper using four items, but it is far from ideal. I have used two adaptors, a step-up ring and a male to male coupler. The step-up ring is M42 x 1 and one of the adaptors is M42 x 0.75, they 'would' fit together with effort, but as I said 'far from ideal' If you offer any solutions can I please ask that the adaptors are 'available' not theoretical, and not from ebay.