Update on 20 July 2012: This is my first time using ND400. What I did was first take a picture without ND400. The scene is the same as the image above. The setting is the same but the exposure time is 1/100s for proper exposure. I multiply the exposure time by 400 (since ND400 is being used) to becomes 4s. Add the ND400 and shoot. I guess I was right. Maybe this will be true for ND8 and ND4.