I have recently bought a D90 and am very happy with it. However, there is one function that I cannot get it to do. It must be possible in such a sophisticated camera but I am baffled by the manuals (pathetic, I know!)

Very simply, I would like occasionally to use only the centre focus point. There must be a way to select this, but I cannot see it. HELP!!

It also occurs to me that it ought to be possible to set the Fn button to select this as a shortcut - is this possible?

Many thanks in advance for the answer to this one.