There's a point midway along the bolt where the threads look deepest and straightest. However, that's not "the central axis of the lens," it's the point where the axis of the bolt is perpendicular to a line drawn to the focal point of the lens. Try this: take a picture of two similar bolts, but put a few sheets of paper under the corner of one, so it leans toward (or away from) the camera a bit. You'll see that the "deepest and straightest" point is displaced upward (or downward).