The main purpose of using neutral density (i.e., ND) filters is to reduce the amount of light that can pass through the lens. As a result, if a shutter speed is kept the same, after adding a neutral density filter, a larger aperture must be used to obtain the same exposure. Similarly, if an aperture is kept the same, after adding a neutral density filter, a slower shutter speed must be used to obtain the same exposure. This can be seen in the following diagram. Note that this diagram was discussed in the Program Mode (950, 990 and 995).
Recall that the thick red line indicates a constant exposure value (i.e., EV). To achieve this “correct” exposure, there are many different aperture-shutter speed combinations. After adding a ND filter, the exposure value is reduced because there is less light passing through the lens. This is shown as a dashed line in the above figure. Thus, if we want to keep the original shutter speed (without using a ND filter), aperture has to be wider; or, if we want to keep the original aperture, shutter speed must be slower.
Different ND filter manufacture many use a different way to indicate the amount of light a ND filter can reduce. There are two typical systems as shown below:
|1/3||2/3||1||1 1/3||1 2/3||2||2 1/3||2 2/3||3||3 1/3||6 2/3||10||13 1/3|
For example, Tiffen and B+W have 0.3, 0.6 and 0.9 ND filters for reducing one, two and three stops of light. Hoya, on the other hand, uses 2×, 4× and 8× to indicate reducing 1 (i.e. 2=21), 2 (i.e., 4=22), and 3 (i.e., 8=23) stops.
All ND filters are gray in color. The deeper the color, the stronger the effect (i.e., reducing more light). The following shows Nikon’s ND4 (font) and ND8 (rear) filters. From the shadows, it is clear that a ND8 blocks more light than a ND4 does.
Based on this understanding, ND filters help us in at least three situations: (1) reduce the intensity of light; (2) use slower shutter speed; and (3) use larger aperture.
Using Slower Shutter Speed
Reducing the intensity of light means we can either use a slower shutter speed or a larger aperture. A slower shutter speed can cause moving objects blurred (i.e., motion blur) which creates a sense of motion. The following images were taken using the Aperture-Priority Mode. The aperture was set to F2.8, the largest possible aperture, so that shutter speed can be reduced properly. The left image below was taken without a ND filter, and, as you can see, the truck (running about 40 miles) is frozen. Motion blur becomes even more significant if ND8 is used (right image below) which reduces the shutter speed to 1/8 of that used for the left image.
|Without ND||With ND filter|
Using Larger Aperture
Since ND filters reduce the amount of light that can pass through the lens tube, they can be used to open up the aperture while keep the shutter speed the same. Keep in mind that a larger aperture produces a shallower Depth of Field. The following images were all taken with the same shutter speed. Using the ND filter reduces the aperture. Now the subject is well isolated from the background, and it shows a better sense of distance.
|Without ND||With ND filter|
|1/160 @ f8||1/160 @ f4|
|1/125 @ f14||1/125 @ f5.6|