Hello, I was trying to create an image that requires a long shutter speed so I went outside and composed the shot (mainly of the sky) and saw that my ND filter could only achieve 1/3rd of a second shutter speed. (note that this was the middle of the day, I was not looking into the sun.) I was looking to get a few seconds at least. Here are the specifics of my equipment: D60 with a Sigma 10-20mm lens. Tiffen 77mm ND filter 0.9 f22 @iso 100. With the filter, shutter speed= 1/3rd. Without the filter= 1/20. Will I have to stack a bunch of these on the lens in order to get, lets say, a 15-20 second shutter speed? How do people get those milky moving clouds? Thanks in advance for the help!