Shooting video in daylight is challenging; it is often so bright out that a high shutter speed on the camera is required. This can lead to jerky and unnatural motion in moving subject, and force the use of high apertures. A solution is to use a neutral density (ND) filter in front of the lens. An ND filter blocks out some of the incoming light allowing a slower shutter speed to be used. Good ND filters attenuate every colour equally (hence the term neutral) while bad ND filters can introduce an unwanted colour cast.
The two main solutions are to use stack of fixed ND filters to control the light level or to use a single variable ND filter whose strength can be adjusted. For my purposes, I decided to go with a variable ND filter. Most variable ND filters are made up of two polarizers that can be rotated with respect to one another. Light travels through space as a wave, and its electric field oscillates back and forth similarly to how a water waves undulates up and down. While water waves can only move up and down, light can oscillated in all directions (for example, side to side, up and down, diagonally, and even in a helical spiral). The direction that light oscillates is known as its polarization.
A single polarizers will cut out 50% (1 stop) of the light from an unpolarized light source like a light bulb. After passing through the polarizer, the light has a definite polarization. If a second polarizer, aligned with the first, is inserted next then all of the polarized light will pass through. If instead the second polarizer is rotated by 90 degrees all of the polarized light is blocked. By rotating the second polarizer in between 0 degrees and 90 degrees the amount of light passing through can be controlled.
The problem with using polarizers to create a variable ND filter is that the polarizers are not always of a high quality. There can be a serious loss of resolution if the optical quality of the polarizer is not good. Poorly manufactured polarizers, with extra materials between them, can also lead to a variable colour cast being introduced into the image–as the polarizers are rotated the colour cast changes.
There are several options on the market for variable ND filters. The deluxe higher end models are the Singh-Ray variable ND filters that can cost several hundred dollars. There is also a newer product out by a company called Light Craft Workshop that makes significantly cheaper (under $100) variable ND filters. I also came across these filters on eBay sold by Rainbowimaging that are incredibly cheap (around $30). I have ordered a number of things from Rainbowimaging before, and have been pleased with the quality and service received. Because they are so cheap, I nearly bought one of these filters just to try out. I had a hard time finding any reviews on them, but did stumble upon some Youtube videos that made it look like these filters introduced a significant colour cast. It was hard to tell if this is the case, but I decided not to risk it. Perhaps one day I'll order one and do a test.
In the end, I went for the Light Craft Workshop Fader ND Mark II (second generation version of the filter). Philip Bloom, a well known blogger and film maker, has good things to say about the Fader ND, so I ordered a 52mm filter from the Canadian distributor on eBay. Again service was excellent and shipping fast. While the filter I ordered will thread onto a 52mm front element, the polarizers are 55mm. This is to help limit vignetting, but means that any other filters/lens hoods that go on in front must have a larger thread size. Every size of the Fader ND uses polarizers that are one step larger.
The Fader ND comes with a convenient carrying case as well as a lens cover (55mm in my case) that works well. The polarizers rotate smoothly and are well constructed. There are marks on the filter to help serve as a rough guide, but in practice I have not found them useful. They do not correspond to the number of stops of light blocked. This is not a fault of the Fader ND, but is a consequence of the way polarizers work. As the angle of the second polarizer changes, the amount of attenuation will not vary linearly. If the Fader ND were to introduce marks that correspond exactly to the number of stops of light blocked, the marks would be distributed in a more complicated fashion. It would be nice to have accurate markings, but for the price I can not complain.
To test the resolution and check for the presence of a colour cast, I carried out two different tests. In the first, I set up a tripod outdoors and shot a sequence of photos of a picture from the book "A New Kind of Science" by Stephen Wolfram. I do not have an Airforce test chart, but this book has very high resolution photos that contain fine patterns suitable for the test I am interested in. I am not able to calculate the resolving power of the lens with and without the Fader ND, but any major loss in resolution should be readily apparent.