I have a D200 that has developed a strange problem.

Very often, as soon as I take up the first pressure on the shutter release, the camera seems to stop-down, which screws-up the metering and the focus. It doesn't do this every time but probably more often than not. Having done it, if I take that wasted shot, the next shot is OK.

It does this, when it does it, on any lens except a fixed aperture cat that I have, where I can here a click that I think is the camera trying to operate the lenses stop-down lever, which of course does nothing on the cat. That's why I think it's a stop-down issue.

Can any one offer any thoughts please ? Thanks.