What is the principle of digital camera polarizer?

Polarizer, also called polarizer, referred to as PL mirror, is a kind of color filter. The excellent function of polarizer is that it can selectively let the light that vibrates in a certain direction pass through, and it is often used to eliminate or weaken the strong reflection of non-metallic surface in color and black-and-white photography, thus eliminating or reducing the light spot. For example, in landscape and scenery photography, it is often used to show the texture of objects with strong reflection, highlight the scenery behind the glass, darken the sky and show blue sky and white clouds. It consists of two pieces of optical glass sandwiched by 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. The coating will be circularly polarized (CPL)

Gradually aging failure, and moisture, impact and vibration will shorten the service life. 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. Light itself is an electromagnetic wave, which is not essentially different from radio waves, but has a shorter wavelength. After the reflection and diffusion of electromagnetic waves, the vibration in a certain direction will be weakened, resulting in polarized light.

A lot of polarized light is harmful in photography. The reflected light on the glass surface prevents us from photographing things in the glass window, the reflected light on the water surface prevents us from photographing fish in the water, the reflected light on the surface of leaves turns the leaves white, and so on. The clear blue sky scatters polarized light in the vertical direction at 90 degrees to the direction of the sun, making the blue sky less deep. If these polarized lights are eliminated, many photos will appear more saturated in color and clearer in picture. A filter that can filter out polarized light is called a polarizer. Ordinary polarizers are called linear polarizers (PL mirrors). Install the polarizer in front of the lens and rotate it carefully to minimize or even disappear harmful glare, so that the polarizer can be used.

I took a photo without glare. If you shoot the blue sky, it will appear bluer and darker. The polarizer not only filters out polarized light, but also filters out the unpolarized light that vibrates in the same direction as the polarized light. Therefore, after using a polarizer, it is generally necessary to increase the exposure of more than one step. However, in AF (Auto Focus) and AE (Auto Exposure) cameras, part of the light will be reflected to the photometric element. If these rays are reflected by a semi-transparent and semi-reflective mirror, and this semi-transparent and semi-reflective mirror is formed by coating with non-metallic materials, then the rays they reflect are polarized light. In this camera, if an ordinary linear polarizer is used, the polarized light passing through the lens will be reflected normally when it hits the semi-transparent mirror at an appropriate angle. If the angle is not appropriate, there will be no light reflection. So AE may be inaccurate and AF may be invalid. In addition, some cameras have LCD screens in their viewfinder. The liquid crystal displays of some cameras are illuminated by light passing through the lens. If the lens uses a linear polarizer, the LCD will fail. Light is an electromagnetic wave. How can the polarizer not only filter out harmful polarized light, but also make the light entering the lens through the polarizer become unpolarized light? In this case, a circular polarizer (CPL mirror) will be used. A circular polarizer can be thought of as consisting of two parts: a polarizer

The former part is an ordinary linear polarizer, which can filter out polarized light in a certain vibration direction. The latter part can make the vibration direction of polarized light passing through it rotate on a circle. This special polarized light is called circularly polarized light, hence the name circular polarizer. For most optical elements, there is no difference between circularly polarized light and unpolarized light, so circular polarizer can be used in any camera. Using this principle, it is easy to distinguish whether a polarizer is PL or CPL. Put the side of the polarizer where the lens is installed close to the eyes, and see the reflection of nonmetal through the polarizer. When the polarizer rotates to a certain angle, the reflection will obviously weaken or even disappear. Do the same experiment with a polarizer in turn, and PL mirror will see the same result, while CPL mirror is similar to ordinary sunglasses. If two linear polarizers overlap and rotate to the vertical direction, the polarized light transmitted through the first polarizer will be completely filtered by the second polarizer, and the effect will be completely opaque. Turn one of them, and the light transmittance will gradually increase, like variable grey mirror (maybe a little color cast)! If one of them is CPL, CPL must be put in the back! If the front polarizer is CPL, you can't see this phenomenon. Many zoom lenses, especially some low-grade zoom lenses, often focus by rotating the front lens. It is very difficult to use a polarizer in this kind of lens: it is necessary to mount the camera on a solid tripod, fix the focus ring with one hand, rotate the polarizer with the other hand, and see the effect in the viewfinder with your eyes. Therefore, if you often use polarizers, especially if you like to shoot scenery, in addition to factors such as price and imaging quality, whether the front film rotates is also a purchase factor. Polarizers are not everything. If the angle of sunlight is not appropriate, the glare reflected by non-metallic surfaces may have a small polarized light component. For example, photographing leaves often cannot eliminate all reflections. If you use a super wide-angle lens, it is difficult to achieve consistent results. Especially when shooting the sky, the direction at an angle of 90 degrees with the sun is very dark, while other directions have little influence, resulting in uneven color and brightness of the sky. When shooting the water surface, some water surfaces do not reflect light, and some water surfaces reflect light very strongly. The polarizer has a rotating part and is relatively thick, which may block the angle when used in a super wide-angle lens. When using a polarizer, don't use a hood because you need to turn the filter. You should try to shoot the camera in the shadow. The prices of linear polarizer and circular polarizer are quite different. Whether the camera can use PL depends on the experiment. First, determine the autofocus: turn the PL mirror to different directions, and press the shutter halfway to turn on autofocus (some fuselage has special autofocus polarizers).

Start button). If it can focus automatically in any direction, it shows that this kind of fuselage can realize AF function with PL mirror. On the other hand, if AF fails in a certain direction, you must buy a CPL mirror! Re-test AE: After installing PL mirror in front of the lens, measure the light vertically against a uniform white wall. Rotate the PL mirror to different directions, and carefully observe whether the luminosity value has changed. If there is any change, you have to buy CPL! If there is no problem with AE and AF, I don't plan to buy another fuselage in the future, I can buy PL mirror!