Thanks Peter.
Whilst I'm not disputing what you've told me, when I bought the lens, the chap at the shop said that the defocusing ring is there to allow me to throw the foreground to subject or the subject to infinity out of focus - which seems more akin to the description of how the Nikon operates. I'm just not sure of the sequence I need to follow to test what actually happens when I engage the defocusing ring.
The defocusing ring has 3 settings. At one extreme it is infinity to around a metre, there's a "click" in the centre at about 1 metre and then at the other extreme it's 0.9 - 0.5 metres. Which sort of begs the question "which setting do I need the defocusing ring to be at if I just want to use it as a standard 40mm wide angle lens"? I presume the infinity end of the scale....?
I've just had a bit of a play with the lens and, despite focusing and defocusing in various sequences, it doesn't seem to make a ha'penny of difference through the viewfinder. Maybe it's more pronounced when stopped down?
I'm intrigued. I'll definitely have to have a play with this lens at various settings and see if there is any difference or if, in fact, the defocus ring is just a decoration!