Found an older article today, but still good information!
“In this article Iâ€™ll tell you why you might use a variable ND filter for video, what it is, and how to use it to trick the exposure metering of the Canon 5D Mark IIâ€™s video mode.”
“So what is a variable ND filter? Itâ€™s a screw-on filter that provides adjustable light reduction. It has a rotating front element (like a circular polarizer), which allows you to control the light transmission of the filter. Theyâ€™re commonly used by landscape photographers to get slower shutter speeds when photographing moving water. The one I have is the Singh-Ray Vari-ND. It provides about 2 to 8 stops of light control depending on the setting.”
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