The neutral density filter, or ND filter, is a powerful tool that photographers use to reduce the amount of light entering the camera lens to select exposure combinations that would otherwise result in overexposed images. This article is a basic guide to ND filters and how to use them.
One of the fundamental principles of photography is light and its effect on the scene. The word “photography”, after all, comes from the Greek words “φως” (light) and “γραφή” (to draw), so it literally means “to draw with light”.
Light, whether natural or artificial, is by far the most crucial element when we photograph as it can transform both the scene and the mood of the image. And it is very important for a photographer to know how to control it according to his needs. But how can a photographer control the light available in the scene?
This is why photographic filters were invented with the first cameras and lenses, because they allow the available light to be controlled according to the needs of the shot.
What is an ND filter?
A Neutral Density (ND) filter is a colorless piece of glass that decreases the intensity of incoming light evenly across all wavelengths – meaning it aims not to affect the colors of a scene – allowing us to have precise control over how much light will enter the camera sensor.
ND filters come in both circular and square types, and they are produced in all sizes, so you are sure to find one suitable for your lens(es).
How do neutral density filters work?
The ND filter sits in front of the lens and essentially blocks some of the incoming light.
A good analogy would be the sunglasses we wear on a sunny day, which helps our eyes see better in bright light.
If you wanted to look at the world with your eyes wide open (like using a larger aperture) in bright sunlight, you could wear sunglasses to block some of that sunlight from reaching your eyes. Otherwise, you’ll have to squint (reduce the size of your aperture) so you don’t hurt your eyes (like overexposing a photo).
ND filters are separated according to their blocking strength, which means that there are ND filters with less blocking strength which allow more light to pass, while others with more blocking strength let pass minimum light levels.
When would you need an ND filter?
The most common situation where an ND filter is very useful is when the shutter speed needs to be extended during the day; for example, to create a nice long exposure of running waters or those pretty clouds passing over the top of the mountain.
Even if a narrow aperture is used, such as f/11 or f/16, there is still plenty of light available and it is not possible to extend the shutter speed beyond one second.
An ND filter, depending on its blocking strength, will allow us to extend the shutter speed to several seconds, as has been done in the example below.
Another situation where an ND filter is useful is when a large aperture (f/1.4, f/2) must be used in daylight but the shutter speed must remain constant; for example, for video shooting or when the light meter warns us of overexposure even though we have reached the fastest shutter speed (i.e. 1/8000). Again, based on the ND blocking strength, we can achieve those nice shallow depth of field shots because the filter reduces the overall amount of light.
Do neutral density filters degrade image quality?
No matter how good and capable your camera sensor is to retain full detail and have high resolution, you should keep in mind that light enters the camera through the lens as well as the filter, which is most often attached to the front.
Depending on the quality of the optical elements of the filter, the final image may have lower sharpness or other optical aberrations, for example flare or even color shift (or color cast) which is often quite difficult to be corrected in post-production.
Of course, this is something that applies to all filters and not just ND, but in the case of ND filters in particular, color shift is a common problem. To avoid having a negative impact on your image quality with your ND filter, buy the highest quality ND filter you can afford.
How to Know Which Type of ND Filter to Buy
As we mentioned earlier, ND filters come in circular and square versions and in all sizes, so it’s easy to find one for your lens. However, they are also separated by their light blocking strength, which is the most important factor when looking for an ND filter to buy.
Besides its brand name (i.e. Cokin, Hoya, Tiffen, etc.), an ND filter often has text and numbers on its product case that tell us about its strength. This text can be something like “ND1000” or “ND3.0” or just “10-stop ND”.
All three of these names indeed refer to a 10-stop ND filter, but each of them refers to something specific.
ND1000 refers to the Filter factor number, which results in the reduced amount of light. In this case, it is 1/1024.
ND3.0 refers to Optical density numberwhich is the amount of energy blocked by the filter.
Below is a table of the most common ND filters that might be useful.
But how can this be translated into real shooting scenarios so that we can better understand which ND we should choose?
Here is another table with some practical data, with examples of shutter speeds.
There is of course the possibility of stacking two separate ND filters – a 5-stop ND and a 10-stop ND to have a 15-stop ND filter, for example – but bear in mind that in most cases and no matter how good the filters are, many optical aberrations can be seen, as well as there is a great possibility of introducing vignetting into your photo.
How to use a neutral density filter
After reaching our shooting location, we start by fixing and finalizing our composition, then we take a few test shots to check the light levels and if the image is correctly exposed according to our creative vision.
We also check if the image is properly focused because even with the latest cameras it can be very difficult to get proper autofocus with a 10-speed ND in front of the lens.
Once everything is checked and finalized, we attach the filter.
To note: Before attaching the ND filter to the lens, we need to make sure it is clean with no visible dust or oil stains that may show up in the final image.
Depending on the blocking strength of the ND filter we have selected, we need to make the necessary changes to the exposure settings based on our initial testing.
So, for example, if our initial baseline (unfiltered) exposure test was aperture: f/8, shutter speed: 1/400 and ISO: 64 and we selected a 10-stop NAexposure settings should be aperture: f/8, shutter speed: 2.5s and ISO: 64 in order to have the same amount of light as the initial shot.
When capturing photos with longer exposure times, we also need to make sure the camera is on a sturdy tripod and will use either a remote control or the camera’s self-timer option to avoid any micro-movement that would spoil our shot. .
Are there other types of ND filters?
Besides the common ND filter, there are also other types of ND available in the market.
Variable ND filters
This is an ND filter which has an adjustable variable density strength, such as ND3-400, which means the blocking strength is not constant, but it can be adjusted by the user according to the shooting needs.
These filters are very handy when you don’t want to carry many different ND filters in your camera bag and are extremely useful for video shooting as the user can adjust the density on the fly. For example, they might be used while filming a wedding and there’s a scene where the couple are outside in broad daylight and then they walk into a much darker building.
Graduated ND filters
The Graduated Neutral Density (GND) filter is a must-have type of ND filter for landscape photographers because they can balance exposure between the bright sky and the foreground, providing a smooth transition of scene light. These filters are mainly available in rectangular form, although there is also a blender (circular) type.
These filters have a transparent bottom portion designed to allow foreground light to pass through unhindered, while the ND portion is located on the top portion to block out some of the light from brighter skies.
Recommended Powers of ND Filters
If you’re into landscape photography, a 10-stop ND as well as a graduated ND is a must to cover most field shooting scenarios.
If you like urban photography (travel, portrait, architecture), a 5-speed ND is probably more suited to your needs. Or you can consider a combination of 3 and 6 stop ND filters which can also be stacked.
If you’re a video shooter, then a variable ND is probably ideal, giving the ability to easily and quickly change blocking force on the fly.
We hope this introductory guide to neutral density filters has been helpful to you on your journey as a photographer. ND filters are powerful tools, especially in landscape photography, and knowing when and how to use them effectively can expand your creative toolbox and take your photography to the next level.
Picture credits: Header photo from Depositphotos. All other photos, unless otherwise stated, are by Christophe Anagnostopoulos