What are the purposes of the sky mirror and deflection mirror?

One. Skylight filter (SL mirror for short) can also absorb ultraviolet rays, but UV mirror can effectively absorb ultraviolet rays, but it has little effect on color balance. In addition to the role of UV mirror, skylight filter can also play the role of color balance. Celestial mirrors are generally divided into 1A and 1B, where 1A is light orange red and 1B is light magenta. These two kinds of sky mirrors not only have the function of UV mirrors, but also absorb 500nm green light (1A) and 550 nm yellow green light (1B) about 10% respectively, so as to reduce the blue phenomenon of the scene caused by scattered light in the sky, and can also correct the blue color of flash photography. Sky mirror is mainly used to adjust the color temperature. Taking color photos in sunny sunshine can get good color reproduction and high-purity pictures. Especially when shooting outdoor portraits on sunny days, it can prevent the reflection caused by the objects next to the characters and keep the skin color of the characters natural and true. In addition, because the skylight does not need to compensate for exposure, it is often installed in front of the lens to protect the camera lens from dust pollution and various possible scratches and bumps. In most cases, the skylight can replace the ultraviolet lens as a protective lens and remain on the lens. Of course, after all, it belongs to the color temperature correction filter, which will be warmer and affect color reproduction to some extent. Personally, it is best to use special protective glasses for lens protection, followed by UV glasses and sky glasses. Because it will change the color temperature, the UV mirror and sky mirror are only part-time protection lenses. If there are no special requirements, it is recommended to choose a UV mirror, which is cheaper in the same filter. Two. 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. 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. 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. Classification of polarizers According to the mechanism 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. The function of PL mirror is to filter the reflected light and increase the imaging contrast. Its working principle is to selectively filter light from a certain direction. By filtering out a lot of polarized light in diffuse reflection, the light intensity in the sky is weakened, the sky is darkened and the contrast between the blue sky and white clouds is increased. Look at the viewfinder and rotate the front mirror when shooting. In the viewfinder, the effect is most obvious when the sky is darkest, and the difference between the darkest and the most appearance is 90 degrees. You can turn to any angle between the darkest and brightest as needed. PL mirror can also effectively weaken or eliminate the reflection of nonmetallic surface, which is a typical polarized light (the light reflected by metal surface is not polarized light, and the polarizer has no effect on it). By adjusting the polarizer, this part of the reflection can be filtered out, thus improving the imaging quality of the subject and the clarity of the picture. For example, using a polarizer can reduce the reflection of the water surface, so that you can clearly photograph the fish in the water. When shooting such a scene, the projection angle of the light source should be close to the projection angle of the camera, and the maximum deflection angle should be between 30 and 40 degrees. When in use, it can be adjusted by slowly rotating the lens in front of the filter to minimize the reflection on the scene surface. In addition, the polarizer can effectively improve the color saturation and contrast, because it can absorb the 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. The polarizer consists of a lens body and a rotatable rear seat frame connected to it. The lens body is made of extremely fine crystal glass. When rotating, the grating of the polarizer blocks polarized light rays that are not parallel to it. Therefore, the polarizer can control and selectively record the amount of reflected light parallel to it (the reflected light is polarized light). This is the reason why polarizer can eliminate or weaken the reflection of nonmetallic surface. But most polarizers are a little blue. At the same time, it will also block some unpolarized light in the same vibration direction as polarized light when using and shooting. In order to avoid the color of the picture being too dim, it is generally necessary to increase the exposure of more than one step. [Edit this paragraph] There are two kinds of polarizers. One is linear polarizer, also called ordinary polarizer, and the English symbol is PL; The other is a circular polarizer, the English symbol is CPL, and the appearance of ol is gray, which can let the linearly polarized light with the same polarization direction pass about 80%, while the linearly polarized light vibrating perpendicular to its polarization passes less than 1%. The appearance of CPL is gray, which is made of a linear polarizer and a quarter-wave plate (a special birefringent material). The optical axis of quarter-wave plate makes an angle of 45 degrees with the vibration direction of polarized light of linear polarizer. Light enters from a part of the linear polarizer in the forward direction and enters from one end of the quarter-wave plate in the reverse direction. The natural light emitted to the circular polarizer passes through the linear polarizer and the quarter-wave plate in turn, and then becomes circularly polarized light. According to the different relative orientations when the polarization direction of the linear polarizer makes an angle of 45 with the optical axis of quarter-wave plate, right-handed circularly polarized light or left-handed circularly polarized light can be generated. At present, the common circular polarizers on the market are Toshiba, Gaojian, Yangao, Baogu, Marumi, Nikon and HAMA. Generally speaking, both linear polarizer and circular polarizer can meet the basic requirements of photography. However, PL will affect metering and autofocus, and is generally not suitable for digital cameras with high degree of automation. Because today's digital cameras have the functions of auto-focusing and auto-exposure, on these cameras, part of the light will be reflected to the metering element. If these rays are reflected by a semi-transparent and semi-reflective mirror, and this semi-transparent and semi-reflective mirror is made of non-metallic material with coating, then the rays they reflect are polarized light. Digital cameras with high degree of automation use ordinary linear polarizers, which will filter out some light information, which may lead to inaccurate AE (automatic exposure) and out-of-control AF (automatic focusing). But CPL works well on this camera. [Edit this paragraph] The skill of choosing polarizer CPL is more expensive than PL. If the camera can use PL, there is no need to buy CPL. Then, can your camera use PL? Just try it. Try AF first: Turn the PL mirror to different directions, and press the shutter halfway to turn on autofocus (some bodies have AF buttons). If autofocus can work normally in any direction, it shows that this kind of fuselage can use PL mirror to realize AF function. On the other hand, if AF fails in a certain direction, you must buy CPL；; Re-experiment AE: After installing PL mirror in front of the lens, measure 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 can only buy CPL, because the polarizer can also reduce the exposure of 1~2, so it can replace ND2 and ND4 neutral gray-scale mirrors in some cases. 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.

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