Understanding the ND Filter
Thomas Lee (Chinese-Canadian Camera Club)
What is an ND Filter?
ND is short of Natural Density. It is a piece of glass (or acrylic) placed in front of a camera lens in order to reduce or modified the light getting into the camera. ND filter is in gray color and there are different filters with different degrees of grayness (ratings). On a camera, a photographer can use the aperture setting and the shutter speeds to adjust the amount of light entering the lens. Putting an ND filter in front of the lens will give the photographer a third tool for adjusting the amount of light.
Ratings of a ND Filter
ND filters are rated with different ND numbers according to their optical transparency, such as ND2, ND4, ND8, ND16, ND32, ND64, ND100 . . . etc. The lager the number, the greater the amount of light will be reduced. ND2 and ND4 are the mostly common used ratings. Also, there is a variable ND filter which allows the user to continuously changing the ND rating without physically changing to a different rated ND filter. By turning the variable ND filter, a photographer can get his precise light intensity for his exposure which cannot be obtained by any f/stop.
Application of an ND Filter
- Extend exposure time without having over exposure take place.
- Blurring the moving water (such as waterfalls, rivers, water waves) by using very slow shutter speeds.
- Use a wider aperture to reduce depth of field (such as portraits shooting) on a very bright sunny day when the required shutter speeds exceed the fastest available on a camera.
- Remove or reduce the visibility of any moving objects (e.g. people on a street or tourist hotspots) by using very slow shutter speeds (up to several minutes).
- In a continuous changing brightness environment, e.g. moving cloud, the photographer can use a variable ND filter to modify the light intensity without changing the camera settings (aperture etc.)
Types of ND Filters
- Screw over the lens, single rating.
- Screw over the lens, variable rating
- Rectangular slot-in ND filter uses with a slot filter holder. The holder can hold multiple filters at the same time to get a mix of rating.
Some brand names of ND Filters:
- B+W http://www.schneideroptics.com/
- Hoya http://www.hoyafilter.com/
- Lee Filter http://www.leefilters.com/index.php
- Singh-Ray http://www.singh-ray.com/
- Tiffen http://www.tiffen.com/tiffen_filters.html