Digital Video - November 2007 - (Page 27) Filters come in four major categories: correction, diffusion/contrast, effect and what I call “special duty.” Correction filters are the most common used and there are two primary types: exposure correction and color correction. Exposure correction, or neutral density (ND) filters, reduce the amount of light passing through the lens without altering the color or contrast. Most video cameras have built-in ND filters, but on some less-expensive cameras these are not actual filters but electronic signal manipulations. Many videographers feel that additional ND filters aren’t necessary, but they do offer a lot more control. Neutral density filters come in various strengths (represented in Log E density) with every three points equaling one stop (1/2 the amount) of light loss: 0.3 is one stop, 0.6 is two stops, 0.9 is three stops and 1.2 is four stops. Many video cameras represent their internal ND filtration in fractions, as opposed to densities, so you’re likely to see 1/16 (1.2 or four stops) or 1/64 (1.8 or six stops). Most video cameras only offer two fixed ND choices, which is why you have more control with external filters. Color correction filters come in many different types, but the two primary filters are the Wratten 85, which converts daylight color temperature to tungsten (5,600K to 3,200K), and the Wratten 80A, which converts tungsten color temperature to daylight (3,200K to 5,600K). All color filters, whether color correction or color effect, work on a subtractive system—meaning they prevent various wavelengths of light from passing through the filter. This means that all color filters cut out a certain portion of light and therefore usually require some kind of exposure compensation. In the case of the 80A, which is a deep-blue filter (cutting out orange, red and yellow wavelengths), there is a loss of two stops. The 85 filter is, generally, only a loss of 2/3 of a stop. It is therefore normally preferable to correct daylight to tungsten rather than vice versa. There are many different strengths of color correction filters, which offer various conversions between color temperatures. In the blue range (converting tungsten toward daylight) you have 80A, 80B, 80C, 80D, 82, 82A, 82B and 82C. In the orange (converting daylight toward tungsten) family you have 81, 81A, 81B, 81C, 81EF, 85, 85B and 85C. Each offers a different amount of color correction, with 85 and 80A being the most correction. Most videographers do not use color correction filters as cameras have internal filters to convert for daylight and tungsten and white balance adjustments for infinite degrees between. Generally I don’t see a need for color correction filters when working with video cameras. (Look for more on color correction and color temperature in future DVD101 columns.) Diffusion and contrast filters are probably the next most common used. There are many different types of diffusion, but they all work generally on the same principle of spreading light or reducing contrast in fine lines. Tiffen’s Pro-Mist filters are speckled with white or black dots that Discussion, questions reflect light passing through to and further informaspread the light into the shadow tion on other types or highlight areas and soften the of filters and uses will be available in the DV.com forums. Visit DV online. www.dv.com overall picture. The Pancro Mitchell Diffusion works a little differently, by refracting the light rays from high contrast areas to diffuse the light. Generally, diffusion filters work to “soften” the image by reducing contrast. Fog filters emulate a fog or mist effect, which also significantly reduces the scene contrast. In addition to the white or black dot diffusions, nets and other textures are available that produce similar results. Contrast filters utilize the same principle by spreading light into the shadow areas to raise those exposures and reduce the overall contrast TO GET THE MOST VERSATILITY FROM YOUR FILTERS, IT’S ALWAYS BEST TO HAVE A MATTE BOX AND A FILTER SIZE THAT EASILY COVERS ALL OF YOUR LENSES. range of the image. With all these filters, the videographer needs to exercise caution—especially with nets or diffusion designed for use with film cameras—as the depth of field is so significant with smaller video cameras that the filter itself is often within acceptable focus and the viewer then sees the texture on the filter rather than the effect it’s supposed to cause. The best way to know what will work is to test various filters with your specific camera and lens choices. Also look for filters specially designed for use with small-chip video cameras. It’s also important to note that various strengths of diffusion filters are required for various focal length lenses. You need less diffusion for longer lenses and more diffusion for wider lenses to achieve the same overall effect. If you start shooting a scene with a wide angle lens and a #1 Schneider Classic Soft, and then move to a longer lens, you might consider bumping down to a 1/2- or 1/4-strength filter to get the same effect as the #1 on the wider lens. Again, testing is your best route to determine what works best for you. Effect filters come in many different types. Color effect filters can emulate day-for-night or a sunset sky, or give your overall scene a Mars-esque red hue. Image effect filters can create star patterns around highlights or a kaleidoscope-like image with multiple views of the same subject. Finally, the special-duty filters like polarizers and color enhancers have specific uses. Polarizers, in addition to cutting a lot of light—generally one and a half to two stops—will help to reduce or eliminate reflections off shiny surfaces from certain angles or serve to increase saturation in a blue sky in certain angles. Polarizers normally only come in a circular variety— whether or not they’ll be used in a mattebox—as they depend on a specific rotation to polarize the angle of light to the lens. Color enhancers or red/green Didymium filters increase saturation of red and green. If you’re photographing a red barn or a forest of green trees, one of these might come in handy to help those colors to really pop. DV dv november 2007 27 http://www.dv.com http://www.dv.com http://www.dv.com
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