Filters aren’t common place in the digital age, Photoshop having covered most ground that filters once did. However there are a few filters that can never be digitally recreated. The Neutral Density filter falls under this category.
The name may sound a little confusing when you first hear it, but its really a pretty simple filter. As the name suggests, it is simply a filter that’s density (read light cutting ability) is neutral all over the surface. In other words, it just cuts out light. Its most common use is when you want a slow shutter speed in bright sunlight. Photographers usually make use of ND filters where they want to depict motion in subjects such as water bodies, and the slowest shutter speed that ambient light allows is still too fast for the cause.
Take for example this photograph of a broken tier at Tank Bund, Hyderabad. The intention was to create a ripple free effect on the water, which is only possible with a multi-second exposure, which was not possible in sunlight. Using an ND filter reduced the shutter speed by 4 stops (from 1/2 sec to 8 seconds), giving it the fine art feel.
ND filters are available in various denominations or ‘stops’.