Working with long exposure images of the ocean near Dana Point California.  I stacked two ND filters and a polarizing filter on top of the ND's.  I set the ISO on my 5D mk2 to 50 or L on the screen to get the longest exposures possible with the ND set up I have.  I need to get another darker ND filter to get the exposure to be even longer.  If you are looking for something new to try with your digital camera, try the ND filters and long exposures.