How to Choose the right ND Filter for More Creative Images

Neutral density filters are often underrated because of the simplicity of their purpose. They're also something of a mystery to many novice photographers. With a little know-how, they're actually very versatile tools. This article will explain what ND filters do and explore some creative ways to use them.

Opening photo: Image taken with the Irix ND32000 filter

...an ND filter is about as simple as it gets...

What a Neutral Density Filter Is

The name might be a bit confusing, but the purpose of an ND filter is about as simple as it gets: it reduces the amount of light that reaches the film or sensor. This is often stated as the light that passes through the lens, however some filters can be mounted at the rear of the lens. Theoretically, a neutral density filter reduces all wavelengths of light equally, hence the name.

There are three basic types of ND filters: solid, graduated and variable. We'll discuss this in more detail later in the post.

Why You Should Want One

You can reduce the amount of light that reaches your recording medium by using a narrow aperture or a faster shutter speed. Heck, you can also lower your ISO setting to compensate for too much light. So, why in the world would you need to add a filter?

The simple answer is to overcome exposure obstacles presented by some situations. For instance, let's say you need to shoot at a wide aperture for shallow depth of field on a sunny day. Even at ISO100, you may not be able to set your shutter speed high enough to avoid overexposure. Adding an ND filter will let you get that bright light under control.

That's only one example. Here are a few other common uses for neutral density filters:

• Blurring motion (such as moving water)
• Enabling a wider aperture setting to avoid diffraction
• Extended exposure times for effects like light painting
• Enabling a slower shutter speed for flash photography
• Reducing the visibility of people or other moving objects from a scene

The list could get much longer, but let's look at a few of those a bit more closely.

Motion blur: Deliberately blurring motion in an image can be used for a number of creative effects. An ND filter can be used to allow you to increase the exposure time to achieve them, particularly outdoors during daylight hours. Some of the most common effects are created by taking long exposures of moving water or clouds.

We'll examine these techniques more closely in an upcoming article on daytime long exposures.

Reducing the visibility of moving objects: Have you ever visited a popular tourist attraction and wished you could remove all the people from your photos? Try adding a 10- to 20-stop filter to your tripod-mounted camera. By lengthening the exposure, people moving in and out of the frame won't reflect enough light to be completely visible. If you're lucky, you can eliminate them completely!

You can also create "ghost vehicles" this way, not to mention unusual "selfies" by walking into the frame, then standing still long enough for the camera to "see" you.

With an ND filter, you can even paint with light during the daylight hours!

Light painting: The fun and fascination of working with moving lights at night can be enhanced by adding a neutral density filter to allow you more time to manipulate the light-emanating objects. This can be helpful in creating "spheres", spirals or similar effects that can take time to "build". With an ND filter, you can even paint with light during the daylight hours!

Light painting and other techniques will also be covered in greater detail in an upcoming article.

Flash sync speed: One of the lesser-known uses for these tools is reducing the Ev of a scene to reduce the shutter speed to the maximum allowable setting. The result of shooting at a higher speed with a focal plane camera will be a black band in your image caused by the shutter curtains.

Filter Ratings

ND filters are rated in several different ways and the differences can be confusing. Here's a basic description of each rating method:

F-stop reduction (2-stop, etc.): This method is pretty straightforward. It simply describes the light reduction provided in terms of f/stops. This helps determine the exposure difference in a way photographers are familiar with. Generally speaking, standard filters are available from 2 to 20 stops of exposure value.

Filter factor (ND2, ND4, etc.): These numbers represent the factor that can be applied to determine the light reduction. They're proportional to full exposure stops, so each number represents double or half the amount of light reduction.

Optical Density (0.3, 0.6, etc.): This rating method is quite common among filter manufacturers. These numbers represent the actual portion of the light that's reduced by a filter.

Transmittance in percent (50, 25, etc.): Instead of representing the reduction of light, these ratings describe the percentage of light transmitted to the camera. While it may seem a bit backward, it's an accurate representation and is often useful in astrophotography.

As you can see, the numbers can get confusing, but the trick is to use the method that works best for you. To better understand the correlation between methods, I highly recommend downloading this free PDF comparison table: Neutral Density Filter Names

Stacking Filters

Multiple neutral density filters can be stacked to increase the overall light reduction. You'll need to use filters or a filter system that allow this, and be aware of how many front-mounted filters can be added before shadows or vignetting occur. Stacking may be easier using a lens with a rear gel filter slot, such as the Irix 15mm and 11mm.

...consider both the density and style of filters.

Choosing Which Filters to Buy

So, with all of the styles and densities available, which ND filters should you have in your bag? Obviously, the answer depends on what you intend to do with them. You'll need to consider both the density and style of filters.

For most general photography, a set of three to five densities will usually round out your kit fairly well. For example, you might select ND4, ND8 and ND16, (2, 3 and 4 stops) to provide up to 9 stops of Ev. For more pronounced effects, you might want to add one or two more to give you 10 stops or more.

Some photographers prefer the simplicity of a variable neutral density filter. This option works somewhat like a circular polarizer, with a rotating front element attached to a stationary rear element. The range of brands and styles available is far too great to cover here. Note that some loss of image quality is extremely common with these.

Front-mounted, threaded filters will need to match the filter size for your lens(es). ND filters are also available in square and rectangular varieties to fit front-mounted, slot-style filter systems. This is the most common configuration for graduated filters, which can be very useful for landscape photographers. It's also a convenient way to stack filters, but your system should be designed to block light from entering between the filters.

Each shot is different and experimentation is the best way to learn.

Choosing Which Filter to Use

So, with all these filter densities available, how do you know which one to use for a given situation? There's no definitive answer for that, since the important part is the result you want.

For instance, a 10-stop filter will let you create awesome effects with moving water, but if you want to simply add a little motion blur and still see the water, you'll want to reduce the filter density. You might only need an ND2 to widen your aperture on a sunny day, or it might take an ND4. Each shot is different and experimentation is the best way to learn. "Chimping" with the LCD preview after each shot is definitely allowed when using these.

It's also important to note that even though a 20-stop ND filter transmits less than 1/10,000 of the light that strikes it, you don't want to use it for viewing the sun, even during a partial eclipse. Neutral density filters are designed to limit visible light, not UV and IR rays.

Focusing and Metering

Focusing with an ND filter attached can be extremely difficult, both for you and your AF system. If possible, it's best to frame and focus your shot with the filter removed, then carefully attach the filter. (Don't forget to turn off autofocus and lock the focusing ring if possible.) Of course, if you're using the hyperfocal scale, you'll have less trouble.

If you're using your camera's exposure metering, there's usually no adjustment necessary, since the light reduction will be compensated for. When you're shooting with very dense filters for special effects, you may need to switch to Bulb mode and use a "best guess" exposure followed by a check of the result.

Setting your exposure manually will require compensation for the filter. You can calculate the exposure based on metering the scene and then add the required number of stops. You may want to bracket your exposures to ensure the best results. Of course, this is a creative process, so the settings you use will depend on what you're trying to accomplish.

Find Your Own Uses and Methods

©Tacha Tassin. All rights reserved.

The ideas and techniques listed in this post are only a small sample of the possibilities with ND filters. As with any creative tool, the only limits are within your own imagination. Wherever that takes you, you'll find that having some neutral density filters on hand will help you explore some new horizons.