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Filters .. Choosing and using materials .. Book of photography

Choosing and using materials

Filters

The filters used in photography are made of a transparent colored material and they absorb part of the light that passes through them. Most of them are made of glass, but we also use filters made of gelatin, which are cheaper but easily damaged.

Filters are usually placed in front of the lens: they modify the image scene by absorbing some of the spectrum radiation. There is a difference in the way the film and the human eye reflect the lights of different colors. For example, in strong light, the eye estimates that the yellow color is the brightest color, while for film, the blue color is the most effective. Filters modify the spectral components of light, as they modify the way in which the film reflects the different colors. In black and white and on film (a sensor for all colors), we can predict the effect of filters by remembering that the spectrum is composed of three color series (three primary colors): blue, green, and red, and that each series between them absorbs the other two series. The basic principle is that a filter brightens objects of its own color and darkens complementary colors. Thus, a red filter transmits red and absorbs green and blue. It transmits objects containing red light in lighter tones, while green and blue colors are transmitted in darker values. In this perspective, orange can be considered a lighter value of red. The most commonly used filter is the yellow filter, which produces a blue sky and shows white clouds. Another advantage is that it slightly reduces the veil. An orange filter is used for a more specific effect. A red filter is more effective and gives a dramatic effect to the image. In contrast, a blue filter shows haze or veil and reduces contrast.

- Filters and their effects:

The human eye and film do not respond in the same way to different colors. For the eye, yellow is the brightest color, while for film, blue is the color. By placing a filter in front of the lens, we adjust the response of any of the colors (detailed drawing) to our liking.

- Filter coefficients:
Filters differ from each other in density and color. The exposure must be increased to compensate for the light absorbed by the filter. The coefficient indicates the value of the increase and the strength of the filter. A filter with a factor of 2x indicates that the exposure must be doubled. Remember that opening part of the pupil equals doubling the exposure. Thus, if the normal aperture is 8 / F without a filter, the absorption of the filter with a factor of 2x will be compensated by changing the aperture to F 5.6. Compensation can also be done by adjusting this time the shutter speed (for a 2x filter, reduce it by half). A filter coefficient does not have a fixed value, but this value varies with the spectral distribution of light. For example, a 4x orange filter requires only a 2x factor in the reddish light of a setting sun. In principle, the bosometer built into the camera behind the lens should automatically compensate for the filter's absorption. In fact, the bosometer's instructions can be wrong because the Cds cell is more sensitive to red at exposure times.

- Polarizing filter:
Light radiation vibrates in all directions of space. Polarized light is formed only from radiation that vibrates in only one plane. A polarizing filter only passes light that vibrates in a certain direction (top image), stopping the vibrations in other directions (below). The effect of a polarizing filter is observed by examining the subject and rotating the filter on itself. The blue sky light in the direction opposite to the sun becomes polarized, which allows the blue sky to be divided into a color image using the correctly oriented filter without modifying the other colors (the filter color is gray - neutral). Reflections on non-metallic surfaces are also a problem from polarized light: they can be reduced or completely eliminated by properly orienting the polarizing filter. The effect is maximum at a reflection angle close to 37°.