What's the use of gray cards in photography?

Judging whether a photographer is professional with a professional eye depends on whether he uses a "standard gray board" to measure light or defines the exposure parameters of the camera. When we mention "standard gray board", we think of neutral gray, because they play an extremely important role in balancing color and gray when recording and restoring color or black-and-white images. To understand the function of neutral gray scale, let's take the exposure meter as an example. Since exposure meters can be divided into illuminance exposure meters and brightness exposure meters, let's start with light.

As we know, the color of everything in the world is presented by the reflection of matter on light. In the study of light brightness, it is found that the reflectivity of colorless snow is about 98%, that of carbon black is about 2%, and the reflectivity of the middle point between them is about 12.5%, so it is called "medium gray". The "standard gray board" is middle gray, and its reflectivity is about12.5%. Through experiments, people know that the 18% reflectivity of matter is very close to the face and the back of the hand of a white or yellow person with bare skin, so photographers use the back of the hand or face as the standard of photometry.

Now, let's go back to the parameters measured and determined by the exposure meter. The exposure parameters obtained by illuminance exposure meter are the basis of correct exposure of camera. We can see that the photosensitivity of the camera not only restores 65,438+08% of the gray level of the subject, but also restores other scenes with different brightness. Here, the illuminance exposure meter measures the illuminance of the light source from the perspective of the subject, while the reading and exposure of the brightness exposure meter means that no matter whether the subject is carbon black or snow, the final result will be restored to neutral gray with a reflectivity of 18%. Among them, the brightness exposure meter is a form of measuring the brightness of the object from the camera's point of view; The illuminance exposure meter has nothing to do with the reflectivity of the subject, and neither of them will get the same photometric parameters because it is white snow or carbon black.

The difference is that the luminometer not only measures the illuminance, but also measures the reflectivity of the object. Under the same lighting conditions, the readings of carbon black and snow are very different. We can use them better only after we understand their respective characteristics in principle.

In addition, the opalescent photometric head of the exposure instrument or the photometric probe of the equipment related to image recording and restoration are used to carry out fuzzy average on the image color, and the final incidence rate is still neutral gray of 18%.

Kodak neutral gray standard gray board for photography

At present, the exposure meters in various cameras on the market are mostly in the form of brightness metering, and their metering readings only provide us with the reference of actual exposure parameters, not the actual and accurate exposure value. The accurate exposure value is determined by the photographer according to the pre-designed shooting effect and the specific situation of the subject. The base point of judgment here does not consider what the measured object is, as long as it is exposed according to the photometric reading, the neutral gray with a reflectivity of about 18% will be finally restored. This "mathematical formula" based on the concept of neutral grey can be used in all stories related to photography. For example, if you want the luminance meter in the camera to have the function equivalent to the illuminance meter, you only need to put a gray board with the reflectivity of 18% under the same lighting conditions as the subject, and the data measured by the gray board is basically the data measured by the illuminance meter. Moreover, whether the tested substance is colored or pure gray, the detection of neutral gray is passed. In practice, the application of standard gray board and neutral gray is not only the precise control and definition of photographic exposure, but also plays an important decisive role in the development and identification of photosensitive materials and post-production of photography. For example, the exposure control of photographic paper by subtractive CMY filter or additive RGB filter in color darkroom is based on the photometric principle of neutral gray, as well as the adjustment of color printing equipment, film photography and camera shooting, the test of digital camera and the calibration of optical instruments. Therefore, no matter in any light, as long as the gray board is placed where the light source of the subject can reach, the imaging equipment can get accurate white balance and exposure parameters!

Here, I want to clarify the concept and definition of 18% standard gray board. When the search for standard gray board spread to photographers with professional thinking, no one actually knew its true definition. Some people spent hundreds of dollars to buy Kodak's standard gray board, some people made it themselves, but people who don't understand the principle and essentials often make the same low-level mistakes. Have you figured out the so-called 18% gray board now? Are you still confused? In fact, the article has an answer, see if you understand it clearly.

As the name implies, "reflectivity" is the efficiency of reflecting light by objects under light irradiation, and 18% of gray matter is determined as the midpoint of full-scale reading of reflectivity of black and white objects measured by photometer after light irradiation, that is to say, this 18% is the nonlinear change value of instrument reading, not RGB = of Adobe Photoshop or other graphics software. In fact, the bottom of the main program interface of Adobe Photoshop and well-known picture browsing software ACDSee is gray, that is, R:G:B= 128 (or R:G:B= 127), while the color number of the webpage is #808080 or #7F7F7F, which is exactly what everyone is trying to get/kloc-. As we know, in the definition of black-and-white image in RGB mode, all image software mathematically defines the uniform linear transition from white to black of RGB = 1: 1 as 0-255; The grayscale mode is defined by a percentage of 0- 100%.