A neutral density filter is a semi-transparent glass filter used in front of a camera lens to reduce or vary the intensity of incoming light without affecting the color rendition. It enables the photographer to choose the exposure time, sensitivity of sensor, shutter speed and aperture size in order to prevent overexposed pictures, motion blur of an object in a wider range and shallower depth of field.
Neutral density filters are quantified either by their f-stop reduction or optical density. They are identified by their light-reducing property, where stronger filters seem to have darker gray shades.
Types of Neutral Density Filters
There are two common types of neutral density filters which include the following:
Variable neutral density filters - These filters are used to overcome the limitation of carrying different filters for each diameter of lens being used. Variable filters are composed of two polarizing filters, at least one of which can rotate. The rear polarizing filter divides the light in one plane. Further, the rotation of front lens cuts the excess amount of remaining light. These filters enable altering the amount of incoming light reaching the sensor.
Extreme neutral density filters – These are multiple stacked neutral density filters used for creating supernal seascapes and landscape with extremely blurred motion. These filters are rated at a 10 stop reduction for allowing slow shutter speeds under bright light conditions.
Neutral density filters are often used to control the exposure time with photographic catadioptric lenses. They also find application in laser experiments where excessive light can damage camera sensors or optical devices. Large telescopes make use of neutral density filters to increase the contrast and reduce the brightness of the planetary pictures.
Another important application is photometry where neutral density filters avoid generation of inaccurate results caused by excess amount of light.
Sources and Further Reading