Polarizer of camera filter

Polarizer, also called polarizer, or PL mirror for short, is composed of two pieces of optical glass with a tiny polarizing crystal (such as mica) with directional function. There is also a manufacturing method. The interlayer between two pieces of optical glass is coated with crystals, such as polyethylene film or poly (vinyl cyanide). This polymer coating can produce a very fine fence-like structure, allowing only light in the same direction as the vibration of the gap to pass through. Then two pieces of glass are independently installed in a rotatable ring, and the polarized reflected light on the surface of the object can be eliminated by rotating one of the lenses. This coating will gradually age and fail, and its service life will be shortened due to humidity, impact and vibration. After all, PL coated with polymer is consumable, not durable. Under normal circumstances, it is no problem to use it for five or six years. According to the different mechanisms of filtering polarized light, polarizers can be divided into two types: circular polarizer (CPL) and linear polarizer (LPL), which have the same function. LPL is mainly used for old-fashioned manual focus cameras. Later, CPL was added with a film with the wavelength of 1/4. This kind of thin film has a special property, which can produce π/2 phase shift on the polarized electric vector in one direction (assuming X), but has no effect on the electric vector perpendicular to it (assuming Y). Therefore, the above polarized light can pass through the 1/4 wave plate along the bisector of X and Y, so the emergent light is a beam of polarized light with vertical polarization direction and π/2 phase difference, which is called circularly polarized light. This design makes it more suitable for new autofocus and automatic exposure cameras. At present, cameras that use AF lens (interchangeable camera lens with autofocus function) all use CPL as polarizer. In some light conditions, linear polarizer may mislead the built-in photometer to measure light, so DC and most autofocus cameras use circular polarizer. Both LPL and CPL contain linear polarizers, but as mentioned above, their structures are different, and their effects on camera photometry are also different. Take a SLR camera as an example. All new SLR cameras have a beam splitting prism device, which sends part of the light to the photometer and the other part to the optical viewfinder. The light entering the photometer is refracted by this prism. If we install an LPL in front of the lens, then for this system, the beam splitting prism acts as the second PL reflector, which blocks the light from entering the photometer, resulting in the photometer getting the wrong exposure value (or aperture value). As for how much light is blocked, it depends on the angular density of prism and PL. For CPL, this will not happen, and the photometer can get the correct exposure value. Which PL mirror to use depends on the type of camera TTL metering system. If it is a prism metering system, CPL should be used, and other metering systems can use any PL mirror. In addition, the effects of the two PL mirrors are the same. Polarizer can effectively improve color saturation and contrast, because it can absorb stray light reflected by fog or dust in all directions in the atmosphere, thus making the photographed image more pure. For example, in the still life photography of flowers, a polarizer is often used to take color photos. It is very useful in landscape photography, flower photography and shooting some specific scenes with strong reflection. When the polarizer is used to take landscape photos, it has an excellent effect on depicting clouds. The refractive index of light in the blue sky is greater than that of light scattered by white clouds. Using polarizer can also make the color of green leaves more saturated and eliminate the fog of shooting urban scenery at low angles. Because the polarizer can also reduce the exposure of 1~2, it can replace ND2 and ND4 neutral gray-scale mirrors in some occasions. It takes some skill to use a polarizer well. For example, when shooting the sky, you can use your right hand to orient your thumb and forefinger at 90 degrees, with your forefinger pointing to the sun and your thumb as the best shooting direction. In addition, because the polarizer will lose the aperture of 1/2 to 2 when the polarization effect is the best, exposure compensation is needed, and generally the exposure of 1 2 will be increased. For the reflection of the metal surface that cannot be filtered, a large polarizer can be added in front of the light source to make the light reflected by the metal polarized, and the reflection of the metal surface can be filtered by the polarizer. However, in portrait photography, it is best not to use a polarizer, because it will filter out the reflection of the face and make the face lose its three-dimensional sense. After talking so much about the benefits of PL mirrors, some people still don't like them. The reason is that it has too many layers, and the two-layer glass lens and thin film are slightly thicker than the general filter. Because of this, the possibility of affecting the light transmittance of the lens (such as dark angle) is increased, which leads to the decline of imaging quality. There are also reasons for product quality. A poor PL mirror is like adding a piece of black glass in front of the camera, 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 great extent, and most of it is linearly polarized. 20% or more of the visible polarized light comes from forests, fields, grasslands and other similar places. The reflected light of mudflats and water often has a polarization of 50% or higher, especially when the incident angle of the incident light is close to Brewster's angle. The light scattered from the clouds has almost no polarization. The degree of linear polarization depends on many factors, such as incident angle, scattering angle, wavelength and weather. It contains a lot of useful information, which people use selectively in remote sensing and photography. Because "there is more than 20% visible polarized light over forests, fields, grasslands and other similar places", in order to highlight its characteristics, the reflected light of these scenes should be used as the main imaging light when shooting similar scenery films. Therefore, rotating PL mirror can image these lights, and at the same time polarize other lights into linear polarization in the same direction, thus weakening their brightness. This is equivalent to improving the contrast of the scene. Therefore, the set of the film should be more beautiful.