Thank you both for replying.
The figure of 20 Mt's i have used is usually a safe approach distance without too much disturbance too a bird, as long as good field-craft is applied, i expect to at least be able to half that distance for smaller birds such as a robin.
Andy in a perfect world that would be the lens for me,as i would like to ditch my scope but unless i win the lottery I'm going to have to settle for less
Rob great information and your pictures give a very good idea of what can be done, very helpful.
It seems a converter is worth investing in and something in the 300-500mm should be what i am aiming for.
All your points are well taken Rob, i will stick to digiscoping for long range birds, and hope by improving my photography skills using my Canon some of it will filter down to my birding side.
brian.