What the Heck is a Neutral Density Filter For?
A neutral density (ND filter) is a special type of filter used to reduce the amount of light entering into a camera’s sensor when you're shooting in bright situations. They also open up an entire world of new creative possibilities to you.
They allow the photographer to select a combination of creative effects such as wider apertures, longer exposure time, extended shutter speed, and sensor sensitivity that would otherwise be impossible without over exposing your image. ND filters are especially useful in bright light conditions to achieve creative effects such as deliberate motion blur, extreme depth of field, or long exposures.
For example, if a photographer wants to create and intended motion blur effect like in the photo of the waterfall above, they may need to leave their shutter open for ten seconds or more.
Without an ND filter, achieving that slow shutter speed in the daytime with all that light reflecting off the water is impossible. Even with an ISO setting of 100 and closing up the aperture, it would be a hard feat. ND filters reduce the light intensity enough to make these shots possible.
How Much Light Intensity Can an ND Filter Block?
Neutral density filters are measured by the number of ‘stops’ of light blocked by the filter.
What are the stops? The ‘stops’ are the measured amount of light. Increasing by one-stop means you are halving the amount of light entering the camera sensor. For every increment of an ND stop the shutter speed needs to be doubled to maintain the same exposure since they halve the light.
Most ND filters are 3-stop filters, 6-stop filters and 10-stop filters. A 3-stop filter allows you to slow your shutter speed to 1/30th per second from a when lighting conditions would normally call for a shutter speed of 1/250th per second. Likewise, a 6-stop filter allows you to reduce your shutter speed from 1/250th of a second to 1/4 of a second, etc.
While some ND filters are labeled by the number of stops of light they reduce, others are labeled with either a filter factor number or an optical density number.
What Do the Numbers on an ND Filter Mean?
Don’t get too hung up by the numbers. For example, a three-stop ND filter may be labeled as an ND8 or ND 0.9 filter, a 6-stop ND filter labeled as an ND64 or ND 1.8 filter and a 9-stop ND filter may sport a label of a ND512 or ND 2.7 filter. Notice as the number of stops of light blocked by each ND filter that the number increases.
Just remember this: the bigger the numbers on an ND filter label, the more light it stops.
When Do I Need an ND Filter?
An ND filter is perfect for landscape photography to capture long exposure effects like the movement of clouds in the sky and fast movement of water.
For example, when you use a low shutter speed of 1/15th of a second or even longer you can create a beautiful, soft effect in moving water. Without an ND filter, achieving a slow shutter speed in the daytime with all that light reflecting off the water is impossible.
Even with an ISO setting of 100 and closing the aperture, it would be a hard feat. But with the ND filter, it reduces the light intensity enough to to allow a long exposure.
Types of ND Filters
ND filters come in several shapes and sizes. Some ND filters are rectangular or square and attach to your lens via a holder. Others are circular or round, but they all do the same thing.
Let's look at my personal favorite filter system first: Lee Filters.
Lee makes the best ND filters out there. I use them personally and recommend highly recommend them.
What makes Lee stand out is that their design allows you to easily stack multiple filters on top of each other to increase your exposure time, or even add a polarizing filter to the mix.
This filter stacking technique is use when one ND filter isn't enough to capture images at the extremely slow shutter speed and wide aperture under dazzling light.
For example, you could stack a 6 stop ND filter with a 10 stop ND filter to reduce light up to 16 stops.
Landscape photographers love graduated filters! These rectangular or square filters have a dark side, and a transparent side. The idea is to place the dividing line between the two right on the horizon, with the dark side lined up with the sky.
This way, you'll capture the detail in the sky that you want, without making the foreground too dark.
Circular Filters: Variable and Solid
Variable filters have two glass rings to let you dial in more or less light intensity. As a plus, you only need to carry one filter. But they do have the drawback as you get to maximums that you can get a cross-pattern on your image.
Solid filters come in different strengths indicated by their stop or optical density number. They allow you to avoid the cross-patterns seen in variable ND filters; however, you'll need more than one.
Honorable mention: Polarizing filters also offer a slight ND effect, reducing light intensity by 2 stops. If you want to increase blue tones in the sky, and slightly reduce light intensity, try using your polarizing filter.
Creative Effects with ND Filters
Here are a just a few of the creative effects that ND filters help you achieve:
Get a Shallow Depth of Field in Bright Conditions – An ND filter will allow you to shoot with a wider aperture than you could normally get away with in bright lighting conditions to create the shallow depth of field that makes your portrait subjects pop against the background.
Capture Movement with Slow Shutter Speeds
A slow shutter speed has a dramatic effect on any photograph that depicts movement. Examples are waterfalls, streams, traffic clouds, smoke, etc.
Want to capture the sun itself? Then ND filters are a must. ND filters with a density stop of 16 or even greater are needed for solar photography. Since most ND filters do not protect from harmful UV and IR radiation, never view the sun directly through your optical viewfinder. Use live view, or your camera's electronic viewfinder (if it has one.)
More on ND Filters
Our friends at B & H Photo have a fantastic article that goes into more depth on various types of ND filters and their pros and cons.
Check it out here: A Guide to Neutral Density Filters.
Have you ever used a neutral density filter? Let me know in the comments.