OK -- I got home and checked mine (which may have a fault). As you push down the button, there's a kind of a scratching noise, and the OVER light flickers (regardless of the light intensity). If the light's too bright, and you push down a tiny bit more, the OVER light stays on. If the light's too dim, and you push down a bit more, the UNDER light comes on. Push down further and the shutter trips.