What is "white balance" in photography terminology?

White balance is a very important concept in the field of TV camera shooting, through which a series of problems such as color restoration and tone processing can be solved. White balance is produced by the true color reproduction of electronic images. It was used in the field of professional photography earlier, and now it is also widely used in household electronic products (household cameras, digital cameras). Although the development of technology makes white balance adjustment easier and easier, many users still don't know the working principle of white balance, and there are many misunderstandings in their understanding.

1. What is white balance?

White balance literally means white balance. Then what is white? This involves some knowledge of chromatics. White refers to the visual response of light reflected to people's eyes because of the same proportion of blue, green and red light and a certain brightness. We all know that white light is made up of seven colors: red, orange, yellow, green, cyan, blue and purple, and these seven colors are made up of red, green and blue in different proportions. When the proportion of the three primary colors of a beam of light is the same, it is customarily called achromatic, and the light reflected by black, white, gray, gold and silver is achromatic. The public's understanding of white is brightness without color components. The white or other colors seen by human eyes are related to the inherent color of the object, the color temperature of the light source, the reflection or transmission characteristics of the object, the visual perception of human eyes and many other factors (please refer to the principles of chromatics). For a simple example, when colored light illuminates an achromatic object, the color of the reflected light of the object is the same as the color of the incident light, that is, the white object is red when illuminated by red light. When two or more colored lights illuminate the achromatic object at the same time, the colors of the object show additive effect. When colored light shines on a colored object, the color of the object decreases. For example, yellow objects appear red under magenta light, green under cyan light, and gray or black under blue light.

Before we understand white balance, we need to understand another very important concept-color temperature. The so-called color temperature, in short, is expressed quantitatively by Kelvin temperature (K). Kelvin, a famous British physicist, believes that if a blackbody substance can absorb all the heat falling on it without loss and release all the energy generated by heat in the form of "light", then it will become different colors due to the level of heat. For example, the thermal stress on the black body will turn dark red when it is equivalent to 500-550 degrees Celsius, yellow when it reaches 1050- 1 150 degrees Celsius, and blue will appear when the temperature continues to rise. The color composition of the light source corresponds to the thermal temperature of the blackbody, and the color temperature of any light is equivalent to the "temperature" when the blackbody emits the same color. This temperature is used to indicate that a certain color light is different from other characteristics, that is, the color temperature. In the process of striking iron, black iron gradually turns red in the furnace temperature, which is the best example of blackbody theory. Color temperature phenomenon is very common in daily life, and I believe people are no strangers to it. Tungsten lamp is yellow because of its low color temperature, and different street lamps also emit different colors. The flame of natural gas is blue because of its high color temperature. The color temperature of cloudless blue sky in Wan Li is about 10000 K, that of cloudy days is about 7000~9000 K, that of sunny days is about 6000 K, that of sunrise or sunset is about 2000 K, and that of candlelight is about 1000 K. At this time, we can easily find a rule: the higher the color temperature, the bluer the light; The lower the color temperature, the redder it is. When the color temperature of a certain color light is higher than other colors, it means that the color light is bluer than other colors, and vice versa; Similarly, when the light of one color is bluer than the light of other colors, it means that the color temperature of this color light is higher, and vice versa.

Because of the unique adaptability of human eyes, we sometimes can't find the change of color temperature. For example, if you stay in a tungsten lamp for a long time, you won't feel that the white paper under the tungsten lamp is red. If you suddenly change the fluorescent lamp to tungsten lamp, you will feel that the color of white paper is red, but this feeling can only last for a while. The CCD adaptability of the camera is not as good as that of the human eye, so if the color adjustment of the camera is inconsistent with the color temperature of the scene lighting, color cast will occur. So what is white balance? White balance is to adjust the color circuit inside the camera to offset the color cast at different color temperatures, which is closer to the visual habits of human eyes. White balance can be simply understood as that the standard white shot by the camera lens is still white after circuit adjustment under any color temperature condition. This is a common situation, but not all. In fact, white balance is to make the light reflected to the lens achromatic by adjusting the internal circuit of the camera (changing the balance relationship of the three CCD levels of blue, green and red). If the white balance is adjusted with reddish light, then the image of this color light is achromatic color's, and the scenery of other colors is blue (complementary color relationship).

Second, the working principle of white balance

White balance is a very abstract concept. The most popular understanding is to make the image of white still white. If white is white, then the images of other scenes will be close to the color visual habits of human eyes. The process of adjusting white balance is called white balance adjustment. There are generally three ways to adjust the white balance of equipment in the early stage: preset white balance, manual white balance adjustment and automatic tracking white balance adjustment. Usually, according to the procedure of white balance adjustment, press the white balance adjustment switch, the white balance adjustment circuit starts to work, automatically completes the adjustment work, and records the adjustment results. If you master the working principle of white balance, it will be more targeted and handy to use.

White balance works like this: there are three CCD electronic coupling elements in the camera, which feel blue, green and red light respectively. By default, the electronic magnifications of these three photosensitive circuits are the same, which are all 1: 1: 1. The adjustment of white balance changes this proportional relationship according to the adjusted scenery. For example, the ratio of blue, green and red light in the adjusted scene is 2: 1: 1 (the blue light ratio is high, and the color temperature is high), then the white balance adjustment ratio is 1: 2: 2, and the obvious blue ratio in the adjusted circuit amplification ratio is reduced, while the ratio of green and red is increased, so that the adjusted scene is photographed by the white balance adjustment circuit. That is to say, if the adjusted white is a little blue, then the white balance adjustment will change the normal proportional relationship, weaken the amplification effect of the blue circuit, increase the ratio of green to red, and make the image still white.

Another way of thinking is to consider the problem of white balance adjustment. The camera will not "reject" the adjusted scene placed in front of the lens within the tolerance range of white balance adjustment, that is, the lens can adjust the white balance of any scene. In most cases, the white palette (card) is used to adjust the white balance, because the white palette (card) can most effectively reflect the color temperature of the environment. In fact, in some cases, the whiteboard (card) is not white, but more or less blue or other colors. Experienced photographers will also use the blue sky to adjust the white balance, so as to get a picture with red and yellow tones. After understanding the working principle of white balance, we will boldly try different effects when using it again, which enriches the camera creation.

Third, white balance adjustment

In life, the color temperature of sunlight is constantly changing. It can be said that no two places are exactly the same. Different regions, seasons, ground environment, weather, morning and evening will affect the color temperature, and there are a large number of artificial light sources, and the color temperature is not the same. There are three methods for photographers to adjust the white balance: coarse adjustment, fine adjustment and automatic tracking (ATW). Coarse adjustment refers to changing the color temperature filter under preset conditions to make the color temperature close to the factory setting. Fine adjustment of 3200K refers to obtaining more accurate adjustment results for the color temperature in a specific environment with the cooperation of the color temperature filter. Automatic tracking refers to the automatic tracking function (ATW) of the camera, and the camera itself can be adjusted at any time according to the color temperature change of the picture.

The preset function is the blue, green and red photosensitive balance set by the camera under the condition of 3200K color temperature. When the ambient color temperature is 3200K, the camera color temperature filter is set at 3200K, which can restore the scene correctly. When the ambient color temperature is 5600K, the camera color temperature filter is set at 5600K, which can restore the scene correctly. When the ambient color temperature is within the range of 3200K 1000K and 5600K 1000K, the color reproduction acceptable to human eyes can be obtained by using the white balance preset function. Because the color temperature deviation is not big, the picture taken shows subtle color changes. This has advantages. Different living environments will have different colors due to the influence of environmental colors and light differences. If white light is adjusted everywhere, different environments will show the effect of single white light illumination, and this rich color change can be preserved by using white balance preset.

The general method of white fine-tuning is to adjust the white balance along the shooting direction with a whiteboard (card) in the shooting environment. This is a common situation, and there are several very flexible adjustment methods for fine white. Use a standard whiteboard with good permeability, put it in front of the camera, and adjust the white balance in the shooting environment against the illumination direction of the light source or the main shooting direction. Professional cameras will give a color temperature reading, such as 5000K K, if you want to shoot a normal picture, use this white balance result to shoot. In camera creation, sometimes we want to get a picture with chromatic aberration to achieve the purpose of creation. At this time, we can adjust the white balance with any scene. The color temperature of the white scene is complementary to the color deviation of the picture, that is, the red and white picture is biased towards cyan, the green and white picture is biased towards magenta, and the blue and white picture is biased towards yellow. The automatic white balance tracking function (ATW) is adjusted in real time with the change of the color temperature of the scene shot by the lens. If the color temperature of the scene shot by the push lens or pan lens changes (the color temperature of the scene shot by the lens is different from the color temperature of the ambient lighting), the color of the picture will change within a lens. If you push the lens from the panoramic view to the close-up of the face, it is not recommended to use this mode unless there are special circumstances, because the color temperature of the lens will vary with the change of the scene and the skin color of the person in the picture will also change.

The last article talked about the concept, principle and use of white balance. White balance adjustment is the basic skill of photography and an important creative means. After the principle is clear, you can use it freely, and you will gain something by groping in practice.