Polarizer, also called polarizer, or PL mirror for short, is composed of two pieces of optical glass sandwiching a piece of tiny polarizing crystal (such as mica) with directional effect. There is another manufacturing method. The sandwich between two pieces of optical glass is coated with a polyethylene film or a crystalline substance such as polyvinyl cyanide. This polymer coating can produce an extremely fine fence-like structure, like a A fine fence only allows light that vibrates in the same direction as the gap to pass through. The two pieces of glass are then installed independently in a rotatable ring. By rotating one of the lenses, the polarized reflected light on the surface of the subject can be eliminated. This coating will gradually age and fail, and moisture, impact, and vibration will also shorten its service life. After all, PL using polymer coating is a consumable product, not a durable product. Under normal circumstances, it will be no problem to use it for five or six years. According to the different mechanisms of filtering polarized light, polarizers can be divided into circular polarizers (CPL for short) and linear polarizers (LPL for short). The functions of these two types are the same. LPL is mainly used on older manual focus cameras. CPL, which appeared later, added a layer of 1/4 wavelength film. This film has a special property that can produce a π/2 phase shift for the polarized electric vector in one direction (assumed to be x), and to produce a π/2 phase shift perpendicular to it. Electric vectors in the direction (let's say y) have no effect. Therefore, the above-mentioned polarized light can be passed through the 1/4 wave plate along the angular bisector direction of x and y, so the outgoing light is a bundle of two polarized lights with perpendicular polarization directions and a phase difference of π/2. This is the so-called of circularly polarized light. This design makes it more suitable for newer autofocus and autoexposure cameras. Currently, cameras that use AF lenses (interchangeable camera lenses with automatic focusing function) use CPL as polarizers. Under some lighting conditions, linear polarizers may mislead the in-camera metering element for light metering, so DC and most autofocus cameras use circular polarizers. Both LPL and CPL contain linear polarizers, but there are differences in structure as mentioned above, and they will have different effects on the camera's metering. Take SLR cameras as an example. Newer SLR cameras have a dichroic prism device that sends part of the light to the photometer for metering, while sending the other part of the light to the optical viewfinder. The light rays entering the photometer have been refracted by the prism. If we install an LPL in front of the lens, then for this system, the dichroic prism acts as a second PL mirror, blocking the light going to the photometer, causing the photometer to get wrong results. Exposure value (or aperture value). How much light is blocked depends on the angular density of the prism and PL. For CPL, this situation will not occur, allowing the photometer to obtain the correct exposure value. Which PL lens to use depends on the type of TTL metering system of the camera. If it is a dichroic prism metering system, CPL should be used. For other metering systems, any PL lens can be used. Otherwise, the effects of the two PL lenses are the same. Polarizers can effectively increase color saturation and contrast. This is because polarizers can absorb stray light from various directions reflected by fog or dust in the atmosphere, making the captured images purer. For example, in photography such as flower still life photography, polarizers are often used to take colorful photos. It is useful in landscape photography, flower photography, and shooting of certain highly reflective scenes. Polarizers are used when taking landscape photos, and have excellent effects on the depiction of clouds. The refractive index of light in the blue sky is greater than that of light scattered by white clouds. Using a polarizer can also make the colors of green leaves more saturated and eliminate the haze in low-angle shots of city scenes. Since the polarizer can also reduce the exposure by 1 to 2 stops, it can replace the ND2 and ND4 neutral grayscale filters in some situations. Using a polarizer well requires certain skills. For example, when shooting the sky, you can use your right hand to align your thumb and index finger at 90 degrees, and point your index finger toward the sun. The direction of your thumb is the best shooting direction. In addition, since the polarizer will lose 1/2 to 2 stops of aperture when the polarization effect is optimal, the exposure needs to be compensated, generally increasing the exposure by 1 to 2 stops. For reflections from metal surfaces that cannot be filtered out, you can add a large polarizer in front of the light source, so that the light reflected from the metal is polarized light, and you can use a polarizer to filter out the reflections from the metal surface. However, when doing portrait photography, it is best not to use polarizers, because polarizers can filter out the reflection on the face, making the face lose its three-dimensional effect. There are so many benefits of PL lens mentioned above, but some people don’t like it. The reason is that it has too many layers. The two-layer glass lens plus the thin film are slightly thicker than ordinary filters. It is precisely because of this that it increases the possibility of affecting the light transmission of the lens (such as vignetting). This results in reduced imaging quality. There are also reasons for product quality. A poor-quality PL lens is like adding a piece of black glass in front of the lens, and sometimes it is even worse. The sunlight shining on the earth is basically not polarized light, but its scattered light is polarized to a large extent, and most of it is linearly polarized. There is 20% or more visible polarized light from above forests, fields, meadows and similar places. Light reflected from mudflats and water often has a polarization of 50% or more, especially when the angle of incidence of the incident light is close to Brewster's angle. Light scattered from clouds is barely polarized.