What are Neutral Density Filters? These are filters that essentially reduce the amount of light that enters through the lens and onto the camera’s sensor. It’s kind of like putting on sunglasses as you go outside on a very sunny day. Now if you understand how a digital camera works (in fact any camera for that matter), the camera takes a photo using the combination of three settings on the camera -- the ISO setting, the focal length, and the shutter speed. By using neutral density filters of different opaqueness, you can control and alter the way the camera would normally take a photo. As you increase the darkness of the filter, you can either increase the ISO, open up the aperture, or slow down the shutter speed.
Why would a photographer want to do this? Primarily to alter just two of the three settings, the aperture and/or shutter speed. By using a neutral density filter you can either increase the aperture opening and decrease the depth of field or slow down the shutter speed and smooth out the effects of moving water or eliminate moving objects altogether from the photo, or both.
Neutral Density Filters are rated according to the number of stops that a filter decreases the exposure on the camera’s sensor. Generally a decrease of one stop is equivalent to an Ultra-Violet filter. They get gradually darker until they result in a 10 stop decrease which is referred to as Black Glass. It’s so dark that you can’t see through the camera with the Black Glass filter attached. All camera settings have to be noted before the filter is mounted and then the camera settings have to be recalculated, adjusted and applied to the camera after the filter is mounted.
To better understand the uses of these filters, I refer you to the Peter Hill’s Ultimate Guide article. Enjoy the information and be sure to scroll down to his illustrative photos to visually appreciate the effects of Neutral Density Filters and especially Black Glass.