I was thinking of picking up some ND filters to enhance my outdoor shooting. I will be returning to some old grist mills soon and would like to be able to slow down the water movement when the sun may be too bright to allow me to use the shutter speed necessary to do it with an unfiltered lens. Is it better to get an assortment of like .3, .6, .9 or can you get something middle of the road like a .6 to serve most purposes? Also do any of you use them w/ outdoor portraits? What about split filters v. ND for landscapes?