After taking my camera outdoors and trying to film I quickly realized the need for neutral density filtering. On a sunny SoCal day you're lucky if you can stop way down to f/22 and get a picture correctly exposed. It's just too darn bright... and of course you really want to be shooting with the aperture open to f/5.6. A reasonable priced solution came in the way of the fader-ND product from Light Craft Workshop.
The filter is a two-part polarizer. You screw in the inner polarizer onto the front of your lens. The outer polarizer freely rotates to give you anywhere from 2 to 8 stops of light attenuation. This was a life saver over the last few weekends of filming. The producer/director wanted to go both indoors and outdoors, which mainly meant just screwing on the filter when we went outdoors and attenuating down the light to get the right exposure. Below is a picture showing the filter at minimal, half, and maximum attenuation. This solution isn't exactly a slam dunk because as you attenuate light there is a slight shift toward green. You have to watch your white balance so the change isn't really noticeable.
After using this tool quite a bit I was wondering why the filter only took a very slight turn to go from over/under exposed to correctly exposed. It turns out the light attenuation isn't really linear (in camera stops). It's more of a logarithmic curve. I measured the actual attenuation by varying the ISO, shutter speed, and aperture on my lens while keeping a white card at 50 IRE (digital value of 128). Below is a graph of the light attenuation with each graduated step of the filter.
As you can see the filter starts at about -1.6 stops of light attenuation (graph is inverted for readability). The LCW documentation claims -2 stops, so with manufacturing margin it's doing what is says. At the maximum filter setting the light is attenuated about -8.6 stops. Again, the LCW documentation says about -8 stops, so they are right. Most of the time when I'm outdoors I need 4-5 stops of attenuation, essentially where the graph's slope starts to get steep. I guess that's why exposure setting is still a bit fidgety while using the filter.
The Singh-Ray Vari-ND is a higher end version of this product and doesn't seem to have the green shift according to others' reports. For the budgets and products I work with the Fader-ND works fine. Both the Vari-ND and Fader-ND are offered in multiple screw mount sizes so they fit many types/brands of SLR lenses. In my case the screw-on filter mounts are all 77mm, so I only need one of these variable neutral density filters. For lenses like the Zeiss CP.2's LCW offers a set of 4x4 matte box filters because the mount on those cinema lenses is just too large. I'm not even sure if those lenses have a screw mount.
Overall it's an essential piece of my camera kit now. It allows me to work faster, better, and produce cinema-like images in just about all filming conditions using standard SLR lenses.