MDJettaTDI

Here is a review that compares many 10-stop circular ND filters for cast and sharpness. Now, I recognize that many here are talking about drop-in filters and holders, but if you want just a straight ND filter and don't want the size and hassle of the holder, circular filters are a viable option. Breakthrough Photography's X3 filter came out on top. http://www.the-digital-picture.com/Reviews/10-Stop-Neutral-Density-Filter.aspx