The principle and use of polarizers
With the popularity of some professional digital cameras, everyone is paying more and more attention to the use of filters. The simplest one is UV mirrors. Its function is to filter out ultraviolet rays (but now everyone is Use it to protect the lens). The polarizers PL and CPL, many newcomers do not dare to involve, always feel that it is too mysterious and difficult to use. In fact, the principle of the polarizer is not complicated, and it is very simple to use. It is one of the most useful filters. Let me give a more comprehensive introduction to the polarizer.
First, the principle
We learned in high school, light is an electromagnetic wave, which is formed by the vibration of alternating electric field and magnetic field perpendicular to the direction of propagation. This wave whose vibration direction is perpendicular to the direction of propagation is called a transverse wave. Sound waves are transmitted by air or other medium compression vibrations before and after. The vibration direction is the same as the propagation. These waves are called longitudinal waves.
The transverse wave has a characteristic that its vibration is polar. It can vibrate in either direction on a plane perpendicular to the direction of propagation. We generally use the direction of the electric field of the light wave as the direction of the light vibration. If a beam of light vibrates in the same direction, we call them polarized light, or strictly speaking, called fully polarized light. Generally, natural light is uniformly distributed in all directions and is unpolarized. However, the glare formed by the reflection of a smooth non-metallic surface at a certain angle (referred to as Brewster's angle, which is related to the refractive index of matter) is polarized light. Deviating from this angle, some unpolarized light is mixed in the polarized light. We call this light partially polarized. Partially polarized light is to a certain extent. The larger the angle of deviation, the less the composition of the polarized light, and eventually it becomes unpolarized light. With polarized light, it sometimes brings us disadvantages. The reflected light on the surface of the glass prevents us from photographing the contents of the glass window. The reflected light on the surface of the water prevents us from photographing the fish in the water. The reflected light from the surface of the leaf sa turns the leaves into white, and so on. The clear blue sky is also polarized in the vertical direction at 90 degrees to the direction of the sun, which makes the blue sky less dark. At this time, we need to use a polarizer.
Spiriting Light Schematic
Unpolarized Light Schematic
2, PL and CPL
Colorless UV Mirror
The polarized light of the lens is shot to half On the mirror, if the angle is right, it will reflect normally. If the angle is not suitable, no light will be reflected, which may result in AE being inaccurate and AF may fail.
In this case, a circular polarizer (CPL mirror) is used. The circular polarizer is made up of a linear polarizer and a quarter-wave plate (for special birefringent materials). The optical axis of the quarter-wave plate forms an angle of 45 degrees with the direction of polarization of the linear polarizer. The light is incident from the linear polarizer at one end to the forward direction, and is inverted from the end of the quarter wave plate. The natural light that is directed toward the circular polarizer passes through the linear polarizer and the quarter-wave plate, and becomes circularly polarized light.
Popularly speaking, a circular polarizer can be seen as consisting of two parts: the former part is an ordinary linear polarizer that filters out polarized light in a certain direction of vibration. The latter portion can rotate the direction of polarization of the polarized light transmitted through it on a circumference. This special polarized light is called circularly polarized light, and the circular polarizer is named for it. Circularly polarized light is no different from unpolarized light for most optical components, so circular polarizers can be used on any camera.
Now that everyone knows the difference between a linear polarizer and a circular polarizer, we can choose whether to buy a PL or a CPL according to our camera.
Three, how to buy a polarizer
Whether a camera can use PL, it is necessary to determine by experiment. First determine the AF: turn the PL mirror to different directions, press the shutter button halfway to turn on the auto focus (some body has a special AF start button). If the autofocus is normal in any direction, this type of body can use the PL mirror to achieve the AF function. Conversely, if AF in a certain direction fails, you must purchase a CPL. Test AE again: After installing the PL lens on the front of the lens, it is metered vertically against the uniform white wall. Rotate the PL mirror to different directions and carefully observe if the metering value changes. If there is a change, you can only purchase the CPL.
The reason for this is because CPL is more expensive than PL. If the camera can use a linear polarizer, there is no need to buy CPL.
When judging the polarization effect, we can observe it through the camera. It can also be observed directly by the naked eye. For example, if you rotate the PL against the LCD monitor, you can see that when you turn to an angle, the PL will hardly reveal the contents of the night view display. Come. The calculator against the LCD also has this effect.
Gray polarizers
Linear polarizers (PL mirrors) are the most common polarizers, but the most common polarizers are not compatible with some highly automated digital cameras. Because today's digital cameras have autofocus and autoexposure features, some of the light is reflected on the photometric components. If the light is reflected by a half mirror, which is formed by coating a non-metallic material, the light they reflect is polarized light. In this kind of camera, if an ordinary linear polarizer is used, in order to prevent the deception of JS, we must recognize which is PL, which is CPL, as follows: the side of the lens mounted lens is close to the eye Look at the non-metallic reflection through the polarizer. When you turn the polarizer to an angle, the reflection will be significantly reduced or even disappeared. Reverse the polarizer for the same experiment, the PL mirror will see the same result, and the CPL mirror is similar to ordinary sunglasses. If the two linear polarizers are overlapped and rotated in mutually perpendicular directions, the polarized light transmitted through the first polarizer will be completely filtered out by the second polarizer, and the effect is completely opaque. Turn one of them, the light transmittance will gradually increase, like a variable gray mirror (may have a little color cast)! If one of them is CPL, CPL must be placed behind! If the front polarizer is CPL, I can't see this phenomenon.
Now, everyone can go and buy PL and CPL with confidence. The better quality brands are Baogu and Kengo. Other brands are judged by the effect of the camera and the rotating feel.
4. Polarization Mirror use method and precautions
The installation of the polarizer is as shown in the figure, which can be directly screwed onto the UV mirror. If a vignetting angle is generated (this phenomenon is especially noticeable on the super wide-angle lens, it is because the lens is relatively thick). Remove the UV and then spin the PL.
The specific adjustment method of the polarizer is as follows: the polarizer is directly mounted on the front end of the lens of the SLR camera or the ordinary DC, and the adjustment ring of the polarizer is slowly rotated while passing through the finder window (the ordinary DC must use the LCD to observe, Cannot observe with the optical viewfinder) Observe the polarized light source in the scene until it disappears or weakens to the desired effect. If the camera does not have an adapter ring, you can't install a polarizer, or you can only view the viewfinder, you can put the polarizer first in front of you and rotate the polarizer adjustment ring while framing until the polarized light disappears or weakens to the expected size. With the adjustment ring orientation remaining the same (ie, keeping the direction indicated by the marker on the adjustment ring constant), the polarizer is translated to fit over the front end of the lens of the camera. After that, the camera should not change the shooting orientation at will, otherwise the polarization direction of the polarizer needs to be re-adjusted.