Introduction to Neutral-Density Filters
Neutral-density filters (also referred to as ND Filters), are an extremely useful tool for photographers and videographers. Surprisingly, many photo/video users have heard of them, but may not fully understand exactly what they're for. In this post, we're going to discuss exactly what an ND filter is and some practical uses for them.
What is a Neutral-Density Filter?
An ND filter is a filter that is grey and neutral in color and reduces the amount of light coming into the lens. The amount of light reduced varies depending on the intensity of the filter used, which is usually measured in full f-stop increments. Different manufacturers have different notations for ND filters, which can sometimes cause confusion when trying to compare. The two most popular notations are the "ND.number" and "NDnumber" - here is a breakdown of how the two compare.
As you can see - for every f-stop, the ND.number rating increases by a factor of 0.3, while the NDnumber rating doubles.
Types of ND Filters
There are a few different types of ND filters. The first and most common are full ND filters. These would cover the entire frame and reduce the light evenly throughout. These can come in screw in types like the 77mm ND Filter 0.6 or square filters for use in matte boxes like the Redrock Micro Filter Set. Some professional video/cinema cameras like the Sony FS7 & Canon C100 Mark II also have their own internal ND Filters that cover the sensor.
Next would be a graduated or split ND - where half of the filter has the ND effect, and the bottom half is clear. This is extremely useful for bright landscapes where you can't evenly expose the entire scene without losing highlight details. Like the full ND filters, these also come in screw-in or square type.
Last is a variable ND filter. These filters allow you to adjust the degree of the ND effect and eliminate the need for carrying multiple ND filters or stacking lower intensities. For example, our SLR Magic Variable ND Filter can provide ND adjustments from 2 to 6 stops.
Uses for ND Filters
One of the most common uses for ND filters is being able to shoot at wider apertures in bright situations. Sometimes even at the lowest ISO and highest shutter speed (between 1/4000 - 1/8000 sec depending on your camera), it's not possible to shoot at wide apertures without dramatically over-exposing the shot. This is especially useful for video users, who are limited by their shutter speed/frame rate combination.
The same also applies for trying to shoot with a flash outside and achieving the flash sync speed (between 1/160 - 1/250 sec). The above picture was shot with a D750, which has a flash sync speed of 1/200 sec. Without an ND filter in this scenario, the maximum aperture you could shoot with would be f/4. Adding a 3-stop ND, we are able to get down to f/1.4 and still be able to incorporate a flash into the shot.
Another common use would be lowering your shutter speed in bright conditions. This is particularly useful for removing the visibility moving subjects in a scene (like a crowded street) or adding a little bit of motion blur to a scene. A perfect example is with water.
In the above sample, the shot on the left does not have an ND filter. This results in a high shutter speed, which freezes the fountain and even the ripples in the pond. The shot on the right however, we added the SLR Magic Variable ND Filter at the maximum setting. You can see now that the water from the fountain looks like it's flowing and the ripples in the pond are much smoother. You can use strong ND filters in landscape situations to create very interesting effects.
That wraps up our intro to neutral density filters. If you're ready to start exploring the power of ND filters, you can find them available for rent on our Filters Section.