For long exposures at or near the 30 sec. Limit of the cameras Auto Exposure capability you should use Manual Exposure mode. In addition Lightning has a short duration and the cameras meter will measure the ambient light level without the light from the lightning included in the exposure calculation. Therefore any images with lightning in it will more than likely be over exposed. Also at night the cameras meter will overexpose for ambient to start with assuming a dark scene providing an image that looks more like daylight than a night scene.