What is a neutral density filter?
- A neutral density [ND] filter, unlike the graduated ND filter, is all one shade of grey with no transitioning from dark to clear. Again, there are several shades of grey with different numbers depending on the shade of grey. (See table below for what the numbers mean).
- The ND filters can be used in at least three separate situations.
- It helps to reduce the intensity of the light coming into the camera.
- The ability to use a slower shutter speed.
- The ability to use a larger aperture. (See depth of field section).
- Reducing the intensity of the light
Depending on the camera, it is possible that a correct exposure cannot be achieved even with the smallest aperture and fastest shutter speed. In such cases, ND filters become useful, because they can reduce the intensity of the light so that a photo can be taken within the limit of your camera. This situation is common when film cameras and high speed films are used.
Using a slower shutter speed
By reducing the intensity of the light you can then use a slower shutter speed or a larger aperture. With a slower shutter speed moving objects become blurred, which then creates a sense of motion. A good example of this is the ‘silky’ effect that you may have seen on flowing water. This effect is achieved with the use of a ND filter and slow shutter speed.
Using a Larger Aperture [smaller f-stop number]
Since ND filters reduce the amount of light, they can also be used to open up the aperture [smaller f-stop number, ie: f/2.8 as compared to f/16] while keeping the shutter speed the same. This produces a shallower depth of field. The following are some examples where this comes in handy.
- Action photography. By panning along with your subject you’re able to freeze the motion of the subject, yet blur the background even further.
- If you are shooting in bright conditions you may find the aperture is too small and the resulting picture will have far too much front-to-back subject sharpness known as depth-of-field. This is often the case with portrait or flower photography where a distracting background ruins the photo. Using an ND filter will help you open up the lens and provide shallow depth-of-field.
- When photographing famous landmarks you often have the problem of tourists getting in the way of your shot. If the shutter speed is slow enough, it will be open long enough to ensure the moving people are so blurred they cannot be seen on the image. A ½ second exposure may record a streak of someone walking, while a 4 second exposure will make them vanish.
What The Neutral Density Numbers Mean
1-stop: This filter will be designated by an ND2, ND2X, or 0.3
2-stops: This filter will be designated by an ND4, ND4X, or 0.6
3-stops: This filter will be designated by an ND8, ND8X, or 0.9
B+W, Cokin, and Hoya use the ‘ND’ designation, while both Lee and Tiffen use the 0.3 to 0.9 designation.
- Also remember that you can ‘stack’ neutral density filters to get in even greater number of ‘stops’ to reduce the brightness coming into the lens.