Seems I can hold it infront of the camera by hand. At low zoom the vignetting is highly obvious when used like this, you just get a circle view in the middle of the picture. You need to get above 10 X to give a clean result but the result looks good enough and the problem might reduce with the proper tube.
8-10 x would be acceptable with some trimming though.
Anyone with an SLR and a suitable lens could also use this. combined with a 300mm it would give a very creditable 600mm amature lens. Of course you could also put another 2X converter between the camera and lens resulting in 1,200mm but I think that would be excessive.
Need to get an adaptor tube though