[greenwing]

Nikon lenses close down to the aperture set on the aperture ring, or to the smallest aperture in the case of G-type lenses (with no aperture ring), when off the camera, by a spring load. When mounted on the camera, they open to their widest aperture by pressure of a camera lever against the lens spring, for metering and focusing. When the exposure is made, the camera's lever moves to the taking aperture position, the lens spring moves the blades to the taking aperture, the shutter opens and closes, and the camera's lever moves back to wide open, forcing the lens wide open again. Take the lens off the camera, and that darned spring closes the aperture down again.

In other words, it's normal, don't panic.

Chris