Originally posted by SpecialK The aperture blades stay open until the camera or your finger closes them by pushing on the lever sticking out the back of the lens.
Just for clarification, older manual style lenses leave the aperture open when the lens is off the camera and while on the camera, will be at the selected apereture setting (unless it has an A setting)
Auto style lenses will keep the diaphragm closed while the lens is off the camera, fully open when on the camera, and when the shutter is pressed (or DoF preview) it goes to the selected aperture.