A neutral density filter was used for this long daytime exposure that caused the clouds to streak and the water to appear dead calm. Copyright Stefan Lozinsky.
You may encounter a photography situation in which the light is too bright for your ISO sensitivity setting or your film speed, preventing you from using a slow shutter speed without overexposing the picture. For example, you may wish to photograph a brightly-lit waterfall at a slow shutter speed, say 1/8 sec. or slower, to intentionally blur the water, but the light is so strong that you can’t use a shutter speed any slower than 1/60 sec. even with your lens stopped down to its smallest aperture. If you have a digital camera, you can try setting its ISO sensitivity to a much lower number, but in very bright light, your lowest setting may not be low enough for very slow shutter speeds. If you are using a conventional film camera, your only alternative is to remove the film from your camera and replace it with a slower speed film, right?
Not so. There is another solution. You can employ a filter that reduces the overall amount of light entering the lens, subduing all colors uniformly, allowing you to use the slow shutter speed that is needed.
Such a filter is known as a neutral density filter or ND filter, also sometimes called a gray filter. It is said to be neutral because it transmits all wavelengths of light equally and therefore has no effect on color. Because of this, a neutral density filter can be used with any digital camera that it fits or either color or black and white film to cut down the amount of overall light striking the digital sensor or the film. Since its density is uniform across the entire filter, light reduction over the entire scene is also uniform.
ND filters are the camera’s equivalent of colorless sunglasses. They simply reduce a scene’s brightness without changing color. They are especially useful in very bright conditions where you would normally wear sunglasses, such as at the beach or on a ski hill under a clear blue sky. They can be used to either permit slower shutter speeds or wider apertures for scenes where less depth of field is desired.
A neutral density filter is used when:
1. you require a shallow depth of field (wide aperture) but your ISO sensitivity setting or film speed is too fast and the light is too bright to permit it;
2. you want to use a slower shutter speed than your film speed or ISO sensitivity setting will allow;
3. avoiding overexposure with a high-speed film or high ISO setting, and
4. shading off the background.
When the light is too bright for you to adjust your shutter speed so that it is slow enough to blur motion, attaching a neutral density filter to the lens will save the day.
DIFFERENT GRADES OF DENSITY
Neutral density (ND) filters are manufactured in several different densities, permitting you to control the amount of light transmitted to the film or your digital camera's sensor. If one grade of ND filter does not quite block the amount of light needed for the exposure settings you wish to use, you can switch to a filter that has greater density and therefore reduces the amount of light even further. A combination of filters stacked together can also be employed to achieve greater density.
Neutral density filters are numbered to designate the amount of light they transmit. For example, a filter with “ND 0,1" stamped on it transmits 80% of the light while the ND 1,0 filter transmits only 10%. The table below shows the gradation range and percentage of light transmitted by various ND filters.
Gradation range and percentage of light transmitted by ND filters.
DETERMINING WHICH GRADE TO USE
In the photography situation described in the first paragraph above, 1/60 sec was the slowest possible shutter speed without a neutral density filter, and 1/8 sec. was the desired shutter speed. The question is, what strength of ND filter should be used to permit a shutter speed of 1/8 sec.?
Here is how to get the answer: Count the stops it takes to change from a shutter speed of 1/60 sec. to 1/8 sec. You'll find there are three - 1/30 sec., 1/15 sec. and 1/8 sec. Now look at the table above, which shows that to open by three stops, you will need the ND 0,9 grade of filter. Therefore, by fitting an ND 0,9 filter to your lens, you would be able to change your shutter speed from 1/60 sec. to 1/8 sec. and obtain correct exposure.
Note: The same information is provided in a different manner by the filter factor. For the ND 0,9 grade, the table shows the filter factor is 8, which means you need to allow eight times more light to strike the film when the filter is attached for proper exposure. Opening by one stop doubles the amount of light, two stops quadruples the amount of light, and three stops permits eight times the amount of light to enter.
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