Not all wide-angle adapters produce barrel distortion. Aspheric adapters are particularly distortion free. The “Bolex Aspheron” (or some-such name) is apparently “the best wide-angle adapter” available. But you have to make your own housing for it and it costs about £420.Tom Hardwick over on knows lots about aspheric lenses. Wittner-Kinotechnik in Germany sell aspheric adapters

There are loads of threads on about wideangle adapters

For my Sony HC7, I’ve opted for the Raynox HD-5050 Pro 0.5x 37mm lens