What is ND Filter? How Does Neutral Density Filter Work?

What is ND Filter?

ND filter (Neutral Density filter) is a gray filter that reduces the light passing through your SLR lens from reaching your SLR’s sensor or film.  The deeper the filter color, the denser the filter and hence the stronger the effect (light reduction). As its name suggests, neutral means it does not affect the color tones/balance on the final image. This is because this uniform-density-filter attenuates the light of all colors (by absorption) indiscriminately and equally. In other words, ND filter’s approach in photography equates to wearing a sun glass in real life, reducing the light passing through the optical glass from reaching our retina.

How Does Neutral Density Filter Work?

As Neutral Density filter (or commonly known as ND filter) reduces the light through the camera lens, our exposure consequently will be affected. In other words, exposure value is reduced with reduced light. In order to attain the same exposure with reduced light, below 2 adjustments are required :

1. Wider aperture
   • Keeping shutter speed as a constant (no change), reduced light means wider apertures is required to obtain the same exposure.
2. Slower shutter speed
   • Keeping aperture as a constant (no change), reduced light means a longer exposure time or slower shutter speed is required to obtain the same exposure.

Manipulating above scenarios, ND is used to allow a wider aperture (to decrease depth of field) or a longer exposure (to create motion blur effect) which is not otherwise attainable within a given range of possible apertures (even at the lowest ISO setting).

When to Use Neutral Density Filters?

Common usage/applications/functions of Neutral Density filters :

1. Enable slower shutter speeds to be used, particularly on high-speed sensors/films. This allows sensors/films to freeze movement of moving objects such as waterfalls, rivers, cars, people et cetera.
2. Enable wider apertures to be used. This decrease Depth Of Field (DOF), allowing user to create the desire creamy bokeh effect.
3. Decrease the effective ISO, particularly on high-speed sensors/films. This allow it to be used even during broad daylight.
4. Allow SLR or camcorder (with fixed shutter speeds) to capture/record bright subjects which can result in over-exposure (such as flickering sand, snow or any other bright scenes).

Flowing Water Motion Blur Effect Using ND8 Filter

In nature photography, ND filter used to record motion (such as swiftly flowing water) as a deliberate moving blur. In above misty, ethereal water image, 8-stops Neutral Density filter was used along with a warm-tone polarizer (to create additional 2 f-stop light reduction), producing a total of 10 f-stop light reduction. It was captured using a 30 second exposure at f16 during broad daylight. If omit the ND filter, the exposure is 1/30th of a second at f16, and the flowing water would not appear like a soft veil.

How Much Light Does Neutral Density Filters Cut?

Understanding how much light Neutral Density filters pass through can sometimes complicated as different manufacturers named them differently. For TianYa brand filter, it is pretty straight forward as it is using filter factor (binary-numbers-based logic). Different notations of ND filters designate the amount of light they cut. A higher grading signifies that the ND filter can block more light, allowing you to use a slower shutter speeds or larger apertures. If you arrange different ND filters vertically and notice the corresponding shadows they produce, it is obvious that a ND8 filter blocks more light than a ND4 filter does.

From Shadow, Notice the Amount of Light A Neutral Density Filter Cut

Generally, there are 2 types of notations used for ND grading :

0.3 ND, 0.6 ND, 0.9 ND, 1.2 ND, 1.5 ND etc

The number listed above suggests the filter density. Each increment of 0.3 designates reduction of light by one stop. This implies that 0.3 ND signifies 1 stop of light reduction, 0.6 ND signifies 2 stops of light reduction, 0.9 ND signifies 3 stops of light reduction and so forth. Lee and Tiffen are using this notation.

ND2(X), ND4(X), ND8(X), ND16(X), ND32(X) etc

This notation is filter factor that based on binary numbers as per formula below:
Filter Factor = 2 ^{f-stop reduction}
2=2¹ indicates 1 stop of light reduction, 4=2² indicates 2 stops of light reduction, 8=2³ indicates 3 stops of light reduction and so forth. Hoya, B+W, Cokin and TianYa are using this notations. ND8 equals to 3 f-Stop reduction.

How to Choose Types of ND Filter?

ND2 and its Usages :
The lightest Neutral Density Filter is ND2 (equal to filter factor 2), which attenuates the light by one f-stop. Example of 1-stop reduction is often used for the right exposure of high-speed sensors/films whenever the subject brightness is still overly high even with the smallest aperture or the fastest.

ND4 and its Usages :
The most popular ND filter in photography is ND4 (equal to filter factor 4) which reduces the light by 2 f-stops. The advantages of ND4 include, for selective sharpness, f/4 rather than f/8 used instead of a great DOF (depth of field), or 1/15 s rather than 1/60 s to get a “flowing” waterfall instead of a “freezing” waterfall. With its fabulous color neutrality, and less pricey than a denser filter, ND4 is often suggested as part of a fundamental photography gear.

ND8 and its Usages :
Corresponding to a filter factor of 8, ND8 is a rather strong, universal gray filter in digital photography. It limits the light up to 3 f-stops. ND8 has more options compared to weaker gray filters and is certainly adequate for effective time-exposure effects. With a ND8 filter, white balance guarantees optimum color neutrality. Hint: For a longer exposure time, don’t forget to make use of your tripod.

ND64 and its Usages :
ND16 filter enables much more intense experiments, attributable to its light reduction by 6 f-stops (equal to filter factor 64). This includes time-exposure effects, for instance, walking people or light trails which emerge as blurred or even invisible. But ND16 actually induces more red transmission which generates a slight warm tone. If needed, this could be removed through white balance as well as an image program.

Is ND Filter Applicable for Me?

The answer is YES if you’re into landscape shooting. One or two Neutral Density filters is sufficient to work wonders to your landscape images. Basically for most waterfall shooting, not more than a few f-stops is required, thus most photographers just equipped themselves with just one or two different Neutral Density filter. 2 and 3 stops ND filters are the most useful and widely-used among photographers. Among other few filters, ND filters produce amazing effects that are not able to be reproduced in the digital dark room by just using Adobe Photoshop or other imaging software.

To buy TianYa filters online at a competitive price in Malaysia,  please visit MY Photo Accessory Online Store.

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