The filter wheel on most professional video cameras contains four different pieces of glass. Number 1 is normally an optical blank. Its primary purpose is to provide an optical path that is consistent with the other three filters--light waves passing through any glass, even optical glass, undergo minor refraction--while allowing all of the light collected by the lens to pass through to the prism block and on to the imaging chips.
Jumping ahead to what is normally filter 3, we find an orange-colored (Wratten 85B) piece of glass. Its function is to remove just enough blue from the image so that the color temperature of a sunlit scene will be reduced from 5600K to 3200K. This color-balancing filter reduces the light by about 50 percent or one f/stop. In addition to color correction, the remaining filters add different amounts of neutral density to the optical path, a necessity when the scene is so brightly lit that the iris alone cannot sufficiently reduce the amount of light reaching the chips.
Knowing which filter to use under a wide variety of lighting conditions allows the savvy videographer to exercise creative as well as technical control over the camera. One way to maintain a little extra depth of field when shooting outdoors under low light is to switch to filter number 1. It's a little known fact that most cameras will white balance just fine without the assistance of a color-correction filter and, since you gain an f/stop without it, you'll have an easier time keeping everything in focus.
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