Filtration, Variable ND

If you’re coming from a background in still photography, one of the first things you’ll notice while shooting video is this: once you lock in the shutter speed, you don’t have nearly the amount of control over exposure that you did back when shooting at 1/1000th of a second was viable. Motion in video will look stroboscopic at higher shutter speeds (see: the action scenes in either Saving Private Ryan or Gladiator), and for motion pictures the standard frame rate is the equivalent of a 180-degree shutter: on 24p cameras, 1/48th of a second, and on 30p cameras, 1/60th. In daylight, if you want to maintain a relatively open aperture (for shallow depth-of-field) at either shutter speed, you’re going to need neutral density (ND) filters. If you’re coming from a background in video, you’re likely used to video cameras having built-in, switchable ND filters; welcome to the world of still cameras, where this is no longer the case. Therefore ND filters are the first thing to add to your filter kit; they come in a variety of strengths, with the most common denominations being 0.3, 0.6, and 0.9 (respectively, 1, 2, and 3 stops of light reduction). You can stack them in a mattebox for greater light attenuation in order to maintain your desired shutter speed and aperture in bright settings.

The above method is the traditional “movie” way of doing things — using drop-in glass filters in a matte box — but in our new era of hybridized imaging, another option is to use a variable ND solution. In this situation, rather than using a drop-in filter or combination of filters in hopes of getting your ideal exposure, you can continuously adjust a rotating filter to darken the scene faster and more precisely.¹ A variable ND setup offers as little as 2 stops and as many as 8 stops of attenuation; the Singh-Ray is the original variable ND filter but the Fader ND series of filters on eBay are just as good and a lot cheaper. Note that newer versions of the Fader ND, known as the Fader ND Mark II (review), alleviate reported image softness problems. What were these problems? Essentially, at longer focal lengths (100mm +), the original Fader ND caused the image to soften — note that this was mostly an issue on still images, where resolutions are several times that of 1080p video. I haven’t experienced any such issues while shooting video with my Fader ND, but I haven’t put it to use as much as I’d like, and I haven’t used it to shoot stills. Note that there are a lot of bootleg Fader NDs on eBay, which are rife with color cast and softness issues — be sure to buy from Light Craft Workshop, the only “authorized” dealer on eBay.

Here is a clip of a variable ND filter in action:

In practice, you wouldn’t do this during a shot; to use a variable ND, you lock in your desired shutter speed and aperture, and if the scene is overexposed (often in daylight), you use the ND filter to bring down the exposure to the correct level. Note that with a screw-in variable ND filter, you will want to buy the size that fits your largest lens (e.g. 72mm) and then use step-up rings on the front of your smaller lenses. A word of caution with using variable ND filters, however: their response to light is not entirely linear, i.e. at higher levels of light attenuation they may let more green light through than other wavelengths. You can see one slight example of this at the end of the embedded video; I would not worry about the Fader for web video and television work as it’s been tested by a lot of shooters (myself included), but if you’re going to the big screen and are very color-critical (and have a sizable shoot budget and the schedule to match), you may want to think about going with the traditional stackable ND approach.

Once you have your ND filters, there is a whole world of creative uses for filters beyond simple exposure correction. This could be its own guide, so for now here is an overview of some different types of lens filters. Note that filters are going in front of your lens, so they are one area where bargain-hunting can be risky; whereas a cheap shoulder support could give you a sore shoulder, a cheap filter could outright ruin your image; beware the reaper of cheap glass.

1. This is achieved by using two polarizers (a circular and a linear) in conjunction with each other; because a polarizer filter only allows light to pass through in one direction (which makes it most commonly useful for eliminating reflections off of windows or water), when stacked with a second polarizer, the two can be oriented so each only allows light through in the opposite direction of the other; therefore, no light is transmitted. [↩]