How to use the grey card

Grey card is suitable for hand-held exposure meters, TTL SLR cameras and paraxial cameras with metering. Using grey cards is the guarantee of taking high-quality photos and a necessary tool for photographers! So, how to use the gray card?

Determine exposure value.

Set the exposure meter to measure the reflected light (remove the white mirror). In order to obtain accurate photometric values, it should be ensured that the illumination of the gray card and the main body comes from the same direction. When measuring values, the distance between the exposure meter and the gray card should be within 15cm, and other strong light should be prevented from reflecting on the gray card or projecting its own shadow on the gray card. When measuring the light in the studio, the gray card should be placed on the front of the subject, as close as possible to the subject. At the same time, the gray surface of the gray card is rotated to form a certain included angle and elevation angle between the gray card, the camera and the light source.

The included angle is generally an isosceles triangle of 30-45 degrees, and the orientation of the gray card should be 1/3 of the included angle between the camera and the main light source. The commonly used lamp position is that the secondary light should be closer to the camera optical axis.

In natural light, you can put the gray card at the position of the subject or at the position of the camera to measure the value. However, it is necessary to ensure that the illumination intensity obtained on the gray card is consistent with the illumination intensity of the subject. If you encounter cloudy days or shoot in poor light, you can use scattered light from the sky, portable external lights or reflectors as light sources to measure exposure value.

In order to make the exposure more accurate, it is sometimes necessary to adjust the camera aperture or speed value after measuring the data.

If the subject is dark in color and tone, the aperture 1- 1/2 should be increased to increase the level of dark parts; If the subject is lighter or brighter, the aperture should be reduced by 1- 1/2. Scenery with normal contrast does not need to be adjusted. However, when shooting scenes with strong backlight or large contrast, it is suggested that photographers adopt the surround exposure method in this case.

Determine the proportion of primary and secondary light

Light ratio is the ratio of the total illumination of main light plus auxiliary light to the illumination of auxiliary light alone. In general, the light ratio of color photos should not exceed 3:1; The light ratio of black and white photos shall not exceed 5: 1. If it exceeds this ratio, it will lead to the imbalance of picture tone and lose the sense of hierarchy.

Using the grey card, the light ratio of the shooting scene can be determined. Determining the light ratio can be performed as follows:

Read the main light value-put the gray card in front of the subject and as close as possible to the subject, and turn on all the main and auxiliary lights (except the backup lights). Adjust the light receiving angle of the gray card in the direction of the main light, and measure the value on the gray card with an exposure meter. When the exposure meter shows the maximum reading, record the reading.

Read the auxiliary light value-the gray card is still placed in front of the main body, turn off the main light and rotate the gray card to align it with the camera lens. Measure the value on the gray card with an exposure meter and record it.

The ratio of scene light can be calculated by obtaining the horizontal difference of two values. The following table shows the ratio of illumination intensity using full light and only auxiliary light.

Determine color balance and density

Gray cards can determine the color balance of photos. When taking a photo, take a gray card into a corner of the photo or take a negative full of the gray card. By comparing the developed photo with the gray card, we can simply judge whether the color of the photo is balanced by the eyes or the color analyzer.

Use the image of the gray card as the reference color for color printing. Shooting a transparent positive or negative of a gray card can help the printer to determine the best color balance and density in his work.

The process of taking a negative of a gray card is the process of recording the color of the scene light. Different light colors will directly lead to different filtering results in color printing.

Most color films cannot display absolute gray values. Therefore, it is difficult to balance the colors from flesh color to neutral color. If the printed gray card is lighter than the neutral color, the color balance of all printed photos can be guaranteed.

Gray card image is the best neutral reference for evaluating color density. You can judge the color density of black and white negative, color negative, positive and developed photos by vision and color densitometer. The neutral gray of gray card is the best color to judge the color balance and film density.

Determine the exposure of close-ups and remakes

Put the gray card on the same horizontal plane of the remake object, adjust the sensitivity of the exposure meter and the camera to the sensitivity of the film used, and measure the exposure value on the gray card along the optical axis of the camera. If the distance between the subject and the camera is less than 8 times the focal length of the lens, the reduction of effective aperture caused by lens elongation should be considered. At this time, it is necessary to increase the exposure and adjust the exposure. TTL cameras can avoid increasing exposure.

The specific conversion formula is as follows:

Determine white balance

Generally, cameras have automatic white balance function, which can cope with most shooting scenes. However, in complex light environment, the automatic white balance function can not completely avoid the imbalance of hue. The white surface of the gray card can reflect about 90% of the incident light. In order to obtain more accurate color, this problem can be solved by measuring the white surface with manual white balance function.

Set the white balance of the camera according to the environment (indoor or outdoor). Aim the camera at the white side of the gray card, then press the white balance key, and the camera records this value. Whether it is a digital camera or a film camera with manual white balance, the white balance of complex scenes can be determined in the above way. Detection camera exposure meter

Grey card can test whether the reading of camera exposure meter is accurate. The following measurements must ensure that the exposure meter is accurate and the test is carried out under the same lighting conditions. Set the exposure meter to measure the reflected light (remove the white mirror). The exposure meter will arbitrarily set a speed value, such as; 1/60 seconds. Then read the reading on the surface of the gray card and record it.

Set the camera's speed value to1/60s, and read the reading on the gray surface of the gray card. If the aperture value measured by the camera is the same as the exposure meter, it proves that the metering system of the camera is accurate. The tolerance between the value detected by the camera and the value detected by the exposure meter cannot exceed 1/3 aperture.

Introduction of existing products on the market at present

At present, there are two kinds of most common gray cards on the market: one is Kodak gray card imported from the original factory. A set of standard Kodak grey cards costs about 180 yuan, including two 8* 10 inch grey cards, a 4*5 inch grey card and a small plastic bag with 4*5 grey cards. The front of the grey card adopts standard 18 degree grey, and the back is pure white.

Another kind of domestic Meinong grey card includes 8* 10 inch grey card and 4*5 inch grey card, and the market price is one set in 68 yuan. It is worth mentioning that, through the research and test of Beijing Minon and the national industry departments, it is found that in practical application, 17.6 degree gray and 88 degree white can be better applied to various complex lighting conditions. Therefore, Meinong Grey Card adopts this reflectivity configuration.

These two grey cards, one is strict with traditional ideas and the other is innovative. Kodak gray card can give a small plastic cover, which is really a very thoughtful design. Mino reduced an extra gray card, reduced the cost, and put it on the market at the price of the original product 1/3, which is undoubtedly the best choice for photographers who pay attention to cost performance.