Neutral Density To The Rescue
So, we have finicky JPEGs, one basic shutter speed, no real exposure latitude to speak of—you can see where this is going. It's an exposure nightmare; you have to be on the money with exposure or your movie is going to suffer.
How does this manifest itself as a real-world pain in the neck? Let me count the ways.
You're outside on a nice bright day and you want to use your large-aperture 70-200mm ƒ/2.8 zoom to blur out the background in a close-up talking-head interview picture. You drop your ISO to its lowest setting, set 1/50th of a second and your aperture is...ƒ/11?
Well, that won't do—not only is the background not dropping out to a nice, even wash of beautiful bokeh, but that nasty speck of dust on your sensor is also manifesting itself, thanks to the extreme depth of field, as a dark blotch in the background.
Cloning out dust or dirt spots with your spot-healing brush just isn't an option in video; remember, that spot is appearing in 24 frames for every second of video you shoot. Oh, you can create elaborate video masks to try and deal with it, but basically, dust spots on a video clip shot at a small aperture are one of the true pitfalls of digital HDSLR video. All the more reason to shoot at moderate to open apertures.
Not to worry, though, because you can banish dust and beautify your bokeh by using neutral-density filters to control your exposure in video. And if you're interested in doing it quickly and easily, those would be the variable neutral-density filters.
I've had these filters in my kit for years, but rarely used them in still work. Now, I use them every time I step outside the door shooting video, however, and I'm eternally grateful for the convenience of being able to dial in the ND strength with the twist of the filter ring, as you do when using a polarizer.
As in most things of an optical nature, the better the glass in your variable ND filter, the sharper your results will be. This becomes extremely noticeable when you're using a long lens. With the explosion in interest in HDSLR video has come a resultant surge in the number of manufacturers offering these variable ND filters.
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