from Ken Rockwell
>>>I carry a Tiffen 0.6 graduated neutral density ("Two Stop ND Grad") in a regular circular screw-in mount. "0.6" is the scientific term (D/log10) for "two stops." You can get it here. (Again, poke around that site to find the size you need.) I have no idea why we call them "graduated" filters, as if they have been granted an academic degree, instead of "gradiated" or "gradiented," which would imply that there is a gradient involved as indeed there is. These are clear glass with one half colored a dark gray and a smooth transitional gradient between the two areas. I only use these only in cases of extreme brightness difference between the sky and ground. This also is a filter that destroys your image if used when it shouldn't be. Photo author Tim Fitzharris and sometimes Galen Rowell use a little too much of these filters for my taste because they turn the sky and mountain tops unnaturally dark. I prefer using a weaker grad filter. I find the square filter systems like the Cokin and Singh-ray too bothersome. Avoid the square systems for rangefinder cameras: they cover up parts of the viewfinder and rangefinder windows, and even worse, one cannot view through the lens anyway to take advantage of the additional flexibility of the square systems on a rangefinder camera. The reason for the one-stop filter factor in the tables below for the two-stop grad for use with rangefinder cameras is because the average factor through the whole filter is one stop (no stops at the top and two at the bottom).<<<