Polariser; Polarizer, Polarizing filter; Polarising filter;
Light passing into the Earth’s atmosphere tends to have a range of polarisations – the waves of light are not neatly aligned. They are chaotic, mixed-up and out of alignment.
Unpolarised light striking a shiny surface (mirror, shiny paint, water etc.) can cause strong reflections. These can overwhelm the ability of the camera to cope with the big contrast between highlights and the surrounding ambient light. It is common for such reflections to be “blown out”. They’re seen as solid white areas on the photograph where the detail is completely lost.
Strong beams of light also create reflections off poorly reflective and diffusing surfaces. This causes an effect where the colours wash out from over-brightness. The sky in particular goes whitish, and often colours on the ground and other surfaces seem to be less intense.
Polarisers, or polarizers, have a specific optical effect as far as photographers are concerned. They filter out some of the light waves at particular angles, but let waves at other angles through. The waves of light that pass through a polarising filter tends to be aligned in one plane. As a result the chaotic reflection from surfaces is reduced which in turn reduces the impact of the strong highlights, and washed out colours appear more saturated.
The alignment effect of this filtration can be seen in the diagram below…
Types of polarisers
There is a range of polarising filter types. In general they fall into two classes. These are the linear polarisers and circular polarisers. Both classes of filter have the same photographic outcome. However, some internal light meters and auto-focus units in DSLRs rely on specific polarisation to work. DSLRs therefore tend to use circular polariser types which do not disrupt meters and focus.
For more detail on how to use polarisers refer to: Circular Polarizing filters.
Polarising filters tend to be made as circular filters with a screw thread to screw onto the front of a photographic lens. The filter glass is mounted in a freely rotating ring attached to the screw-thread section. This allows the filtration glass to be freely rotated without tightening or loosening the fixing in the lens.
To adjust the polarisation effect: The rotating part of the filter should be rotated (in either direction) until the desired light reduction / polarisation / colour saturation effects are apparent.
The polarisers used in photographic situations aligns the light wave in one plane reducing the light passing through the filter. The rest of the light is reflected off the filter or is absorbed by the filter (creating heat).
Polarisers typically reduce the light entering the camera by one to three stops. They are therefore seen to have additional photographic benefits from light reduction in very bright light. They may replace a Neutral Density Filter if the photographer needs only moderate light reduction.
Polarising lenses are also commonly found in sunglasses. They have the same optical impact as when mounted on a photographic lens. They can be used as a useful guide as to the effect of polarisation when considering the set-up of the filter/lens assembly.
Photographically, polarising filters act to filter light and are therefore optical filters. However, they can be made for other electromagnetic radiations/waves. They can be considered as a class of generalised filters – not purely optical in nature.
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