How to Choose a Neutral Density Filter Follow
There are three predominant types of neutral density filters.
The first, and historically most common, are of fixed density that screw onto the filter ring of the lens in use. These typically do not negate the use of lens hoods and even very good ones are still relatively inexpensive. They are easy to use, except when you get to high densities that may cause problems for the light meter or the autofocus system.
The second method involves attaching a filter holder and using either glass or polycarbonate square or rectangular filters that slide into the holder. We first saw this with the Cokin system, and today we have many choices in this regard. Another advantage of this type of system is that there will be mounts available to use on lenses with bulbous front elements that cannot take a standard filter.
The third option is the variable neutral density filter. A variable neutral density filter is two polarizing filters mounted together. By turning the outer filter, you can vary the level of light reduction in general by 1 to up to 8 stops depending on the brand. A high quality, variable neutral density filter should cost about twice as much as a good polarizer.