Very true Simon also bear in mind that these work better on leses above 35mm..  This fact is why the 25mm extension tube won't work with anything shorter than a 35mm lens because the focus zone is moved in so close the front glass element on the lens prevents you from getting that close.

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt