One of the oldest methods of customizing the image is in the use of filters usually placed in a filter holder/matte box positioned in front of the lens. These are used for various reasons such as to control light, contrast or part of the subject brightness, to soften the image or to colour the image. Most camcorders also have one or two filter wheels fitted between lens and prism block carrying colour correction filters and/or neutral density filters.
Altering the appearance of the image
Filters fall into three main groups – colour correction filters, neutral density filters (used as a control in exposure), effects and polarizing filters. They all alter the quality of light reaching the CCDs, but whereas the first two groups attempt to invisibly make the correction, effects filters are intended to be visually obvious in their impact on the appearance of the image. They are employed to change the standard electronic depiction of a subject. Filters can be chosen to bring about a number of visual changes including reduction in picture sharpness, a reduction in picture contrast, lightning or ‘lifting’ blacks, to induce highlight effects such as halos or starbursts and to modify the rendition of skin tones. Many of these effects are not reversible in post production although improvement in picture matching can be attempted.
Factors that affect the filter influence
Many of the effects filters such as black and white dot, frosts, nets, fog, soft and low contrast achieve their results by introducing a varying degree of flare. The potential influence on the picture is identified by a grading effects filter on a scale of 1 to 5 where 1 has the smallest effect and 5 has the largest. Some filters are also available in smaller, more subtle increments and are graded as 1/8, 1/4 or 1/2. Choosing which grade and which filter to use is dependent on the shot, lens angle, aperture and the effects of under-or over-exposure. In general, effects filters work more effectively on a longer lens and wider apertures (e.g. f2.0 and f2.8). To ensure continuity of image over a long sequence of shots it may be necessary to vary the grade of filter depending on lens angle, camera distance and aperture. It is prudent to carry out a series of tests varying the above settings before production commences. Filters mounted on the front of the lens are affected by stray light and flares which can add to the degradation.
Filters that affect the blacks in an image
A strong black makes a picture appear to have more definition and contrast. Diffusion filters reduce the density of blacks by dispersing light into the blacks of an image. This effectively reduces the overall contrast and creates an apparent reduction in sharpness. White nets provide a substantial reduction in the black density and whites or any overexposed areas of the image tend to bloom. If stockings are used stretched across the lens hood, the higher their denier number (mesh size) the stronger the diffusion. A fog filter reduces black density, contrast and saturation. The lighter parts of the scene will appear to have a greater fog effect than the shadows creating halos around lights. A double fog filter does not double the fog effect and possibly creates less of a fog than the standard fog filter, but does create a glow around highlights. Because of lightening of the blacks, the picture may appear to be over-exposed and the exposure should be adjusted to maximize the intended effect. Low contrast filters reduce contrast by lightening blacks and thereby reducing the overall contrast. Strong blacks appear to give the image more definition and low contrast filters may appear soft. They are sometimes employed to modify the effects of strong sunlight. Soft contrast filters reduce contrast by pulling down the highlights. Because blacks are less affected, soft contrast filters appear sharper than low contrast filters and do not create halation around lights. As highlights are reduced the picture may appear under-exposed.
Effect on highlights
Some filters cause points of light, highlights or flare to have a diffused glow around the light source. The black dot filter limits this diffusion to areas around the highlights and avoids spreading into the blacks. Super frosts, black frosts, promist, black promists and double mists diffusion work best on strong specular light or white objects against a dark background. The weak grades leave the blacks unaffected providing an apparent sharp image whilst the strong grades cause a haze over the whole image, which leaks into the black areas of the picture. Black, white and coloured nets have a fine mesh pattern causing a softening of the image and a reduction in the purity and intensity of the image’s colour. This ‘desaturated’ look is thought by some to give video images more of a film appearance.
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Neutral density filters: These reduce the amount of light reaching the lens and can be used to produce a specific f-number and therefore depth of field. |
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Polarizing filters: These reduce reflections and glare, darken blue skies and increase colour saturation. They are useful in eliminating reflections in glass such as shop windows, cars and shooting into water. The filter must be rotated until the maximum reduction of unwanted reflection is achieved. This changes the colour balance (e.g. can affect the ‘green’ of grass) so a white balance should be carried out when correct filter position has been determined. Moving the camera (panning or titling) once the polarizing filter is aligned may reduce or eliminate the polarizing effect. |
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Star and sunburst filters: These produce flare lines or ‘stars’ from highlights. Star filters are cross hatched to produce 2, 4, 6, 8 or 10 points whilst sunburst produce any number of points. They are more effective when placed between lens and prism block and can produce an unwanted degradation of definition when in front of the lens. |