Neutral Density Filters for Video

Hello readers!

We are all aware of what a neutral density filter does for still photography. There are two primary use cases.

The first is to reduce the amount of light entering the camera to allow for longer shutter speeds in order to achieve selective motion blur. Think waterfall image.

The second is to reduce the amount of light entering the camera to allow for a larger aperture in order to deliver very shallow depth of field. Think portrait outdoors on a bright day

NDs for Video

Cinematographers used neutral density filters for a very long time for a very simple reason. The ISO / ASA of the film stock was fixed, and the cameras had a stock shutter angle, leaving the only variable the iris. Still photographers call this the aperture.

The filmmaker always wants full control of depth of field. When we move to digital filmmaking aka video, that does not change. We want to maintain a consistent ISO so the look of the different clips does not change when we go from an inside shot to an outside shot. We also shoot typically as a single frame rate (may be two) which looks down the shutter speed as I have explained in past articles. Thus in practical terms, only the aperture can vary, yet as videographers, we often want to explicity control depth of field, particularly for close up and medium shots. If I am shooting a scene outdoors on a bright day where I need shallow depth of field, the only option that I have is to add a neutral density filter.

I may already be at the optimal ISO and have defined my framerate which fixes my shutter speed. I can get to the desired exposure by manipulating the aperture, but I am better served by using a neutral density filter to reduce the light hitting the sensor to I can use the depth of field that I want for the shot.

This demand is sometimes fulfilled in pro video cameras with built in physical ND filters. Some products use a digital ND which is really a gain reduction tool, not a filter at all. Some shooters just overexpose and try to adjust in post processing. For me, and for the majority a set of ND filters is the most inexpensive and best looking way to achieve the exposure and the depth of field that you want.

What About Variable ND Filters?

This demand engendered the release of multiple “variable ND filters” about the same time when the world figured out that the then current Canon 5D Mark II was a superb camera for video. The challenge is that a variable ND is actually two polarizers stacked together. One is fixed and the other rotates in front of the first one and this is what produces the variance. Yet serious image makers know that there is a massive difference between a crappy polarizing filter and a really good one. The majority of the variables cost less than a generic polarizer, so we know what kind of quality we were getting and it was proven on the first shot. Loss of saturation, colour casts and softness were common and very correct complaints. These days most folks have figured out the variable NDs are a bad choice overall, even if you spend the money on a really excellent one.

Mount Types and ND Rating

Whether you choose the screw in type or the type that are square or rectangular that slide into a holder is up to you, although the lens design may force you in one direction.

Look to building a selection of ND filters in two stop increments at the end. This is why some pros go around with a filter box. I would start with a 4 stop and an 8 stop, then add a 2 stop, a 10 stop and finally a six stop. Given the dynamic range of our sensors, this is going to be a very flexible kit and suitable for video and also for still. Go with a higher quality brand and you will be in good stead. I have used B+W, Heliopan, Lee, Haida and Breakthrough filters for both purposes and been happy with any of them.

For video, colour accuracy is important of course, but if you have seen any film shot in the last ten years, you have also seen the impact of a what is called a LUT. A LUT very simply is colour model for highlights and shadows. Some LUTs are used to convey an emotion such as the vaguely greenish feel of anything happening in The Matrix, or the warmth of footage shot in hot and humid Florida having an orange cast to the highlights and a cyan cast to the shadows. As LUTs are added in post anyone using a LUT is going to be altering the perceived white balance anyway, so don’t panic if you think that you see a slight colour cast in an ND. These casts will be most evident in denser NDs, and are easily corrected in post processing. The casts which change over a shot caused by a variable ND are much harder to fix, another reason to avoid variables.

Thanks very much for reading. Please subscribe to be notified of new articles and podcast episodes. Until next time, peace