Almost all light sources, whether natural or artificial, emit non-polarized light. However this light, if it is reflected from certain surfaces or passes through the atmosphere and therefore spreads, is polarized, i.e. oriented from the electromagnetic point of view on a single axis. This polarization process can be complete or partial, so it can affect the entire light beam or only part of it. In any case, the function of the polarizing filter is to block polarized light.
In practice, a polarizing filter allows you to:
- Reduce or eliminate reflections from non-metallic surfaces. The polarizer can prevent the camera from mirroring on glass or the sky from reflecting on a windshield; it can make the bottom of a river visible and cause the blades of grass in a meadow or the leaves of a tree to appear bright green, losing the intense reflection of the sun.
- Darken the sky. The polarizing filter offers the maximum darkening of the sky when the sun is at a height of 45 ° above the horizon and 90 ° with respect to the camera axis, i.e. the extreme right or left of the camera. The effect is greater in clear newspapers as the water particles that make up the haze can have a depolarizing effect. Shooting against the light minimizes the possibility of polarization and those with the sun behind them greatly reduce it.
- Reduce the incidence of haze. The polarizer, based on the angle of the sun's rays, can make shots of landscapes and in general of long-distance subjects more contrasted and clear, reducing reflections on water particles in the air. By eliminating some of the reflected light from the water, it can happen that images taken through a polarizing filter are particularly warm in terms of color balance.
- Saturate the colors. By eliminating the polarized component of light from the formation of the image, it is common to obtain decidedly denser and more saturated colors, but often also more flat and opaque.
Since the effectiveness of the polarizer is linked to its orientation with respect to the light, to use the filter it is rotated until the desired result is obtained, bearing in mind that the intensity of the polarization goes from the minimum to the maximum in an arc of 90 °.. By rotating the filter by 360 ° the two extremes of the polarizing effect are then reached twice. The effect of the polarizer is also visible to the naked eye, just look through the filter as you rotate it. The more intense the effect becomes, the less light is allowed through the filter.
Since the effects of the polarizer are largely unrepeatable in post production, this filter is still one of the most used in the film industry today. When it comes to outdoor daytime shooting it is more common to see it in use than not. However, since the effect of the polarizer varies according to its position relative to the light source, it is good to be very careful when using it in daylight as it may be difficult to maintain visual continuity. For the same reason it is necessary to pay particular attention to shooting in motion, as working with a polarizer could cause unnatural variations in the brightness and color of the images.
A final usage note: certain modern screens, such as LCDs, emit polarized light and may appear dull when viewed through a polarizing filter.
There are two types of polarizing filters:
- circular polarization (marked by the initials C-PL or C-POL )
- the linear polarizer (marked by the letters PL, L-PL, POL or L-POL )
Linear polarizers are now quite rare as they are incompatible with phase contrast autofocus systems and with the exposure metering of reflex cameras. These limitations are the reason why the circular polarizer was invented. The indications of linear and circulartherefore regardless of the shape of the filter, which in both cases can be round or rectangular. What distinguishes the two filters is the type of polarization they apply; different in method but equally effective. Both filters therefore perform the same function and therefore reduce the amount of light that reaches the lens, without however changing the color, if not due to the lack of neutrality of the filters themselves, due to the materials that compose them. Generally, the reduction in brightness made by a polarizing filter fluctuates between 1 and 2 stops. The differences between an inexpensive polarizer and a high-end one reside both in the quality of the glass and the surface treatments (and therefore in the possible chromatic shift introduced) and in the polarization capacity, which in the best models can reach 99.5%.
Source: https://www.dzofilm.com/
