Most landscape photographers will tell you that having an ND (neutral density) filter or two in your kit is an absolute necessity. There are many choices on the market, and it's often difficult to figure out what is what in the world of ND filters. So lets take a look at one of the better offerings and what exactly an ND filter allows a photographer to do.
At the most fundamental, a neutral density filter reduces the amount of light hitting the sensor, causing shutter speeds to increase in proportion to the number of stops that the filter cuts out. There are graduated ND filters, which allow for selective adjustment of extremely high contrast scenes and result in a reduction of overall contrast in the final image. More recently variable ND filters have become more common. We will look at the Promaster variable ND filter with a range of ND3-ND400 which basically means an approximate 2 stop-8 stop reduction in exposure. Now to put some numbers to these values, if your measured shutter speed is 1/2000th, this filter could make your shutter speed as slow as 1/10th of a second. Additionally, if your shutter speed is 1 second, this filter could help you arrive at more than a 6 minute exposure at the same aperture! This is huge, when desiring to record fast moving water or slow moving clouds over a longer period of time. Having the flexibility to adjust the exposure by simply rotating the filter allows you to adapt quickly to changing light conditions and to create a wide range of images at a single location.
When traveling in areas where running water is of photographic interest, definitely have one of these in your bag!
UPDATE: In recent testing with a new ProMaster ND filter that I purchased to fit a new lens, I discovered that the copy I received was defective in construction. This issue caused a significant loss of sharpness in the resulting image. Make sure that you test your gear before hitting the road on your travels! I'm certainly glad I did. I'm working on an exchange now.