Introduction and usage of grey card

The gray card is the benchmark for accurately detecting the exposure. Photos with rich layers and saturated colors come from accurate exposure. If the light of the shooting scene is too bright or too dark, it may lead to exposure deviation. The gray card can balance the complex light scenes into 18% neutral gray. By recording the reflected light of the gray card through exposure meter, you can get accurate exposure values!

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

Determine the exposure value

Set the exposure meter to the function of measuring reflected light (remove the white reflector). In order to obtain accurate photometric values, it should be ensured that the illumination of the gray card and the subject comes from the same direction. When measuring values, ensure that the distance between exposure meter and the gray card is within 15cm, and avoid other bright light from reflecting on the gray card or projecting its own shadow on the gray card. When measuring the studio light, the gray card should be placed on the front of the subject and 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.

generally, the included angle is an isosceles triangle of 3-45 degrees, and the orientation of the gray card should be directly opposite to 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 optical axis of the camera.

In natural light, the gray card can be placed 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 a weak light environment, you can use scattered light from the sky, portable exterior lights or reflectors as light sources to measure the exposure value.

in order to make the exposure more accurate, sometimes the camera aperture or speed value should be adjusted after the data is measured.

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

determining the ratio of main light and auxiliary light

The light ratio is the ratio of the total illumination intensity of the main light plus auxiliary light to the illumination intensity of the auxiliary light alone. Under normal circumstances, the light ratio of color photos should not exceed 3:1; The light ratio of black and white photos should not exceed 5:1. If it exceeds this ratio, it will lead to the imbalance of picture tone and the loss of hierarchy.

use the gray card to determine the light ratio of the shooting scene. Determining the light ratio can be carried out 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 (excluding the reverse light source). Adjust the light receiving angle of the gray card towards the main light direction, and measure the value on the gray card with exposure meter. Record the reading when exposure meter shows the maximum reading.

read the secondary light value-the gray card is still placed in front of the subject, turn off the main light and rotate the gray card to align it with the camera lens. Use exposure meter to measure the value on the grey card and record it.

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

determine the color balance and density

the gray card can determine the color balance of the photo. When taking a picture, shoot the gray card into a corner of the picture or shoot the gray card all over a negative. By comparing the developed photo with the gray card, we can simply judge whether the color of the photo is balanced by eyes or color analyzer.

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

the process of taking a negative for a gray card is the process of recording the light color of the scene. The difference of light color can directly lead to different filtering results in color printing.

most color films cannot show absolute gray values. Therefore, it is difficult to balance the colors from flesh color to neutral color. If you print the gray card a little lighter than the neutral color, you can ensure the color balance of all printed photos.

The image of gray card is the best neutral reference for evaluating color density. It can judge the color density of black-and-white negative, color negative, positive film and printed photos through 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-up and remake

place the gray card on the same horizontal plane of the object to be remake, adjust the sensitivity of exposure meter and 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 the effective aperture due to the extension of the lens should be considered. At this time, the exposure should be increased and adjusted. TTL camera can be used without considering 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 9% 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 to the gear position of the environment (indoor or outdoor). Aim the camera at the white side of the gray card, and then press the white balance key, and the camera records this value. Both digital cameras and film cameras with manual white balance can determine the white balance of complex scenes in the above way. Detecting the camera's exposure meter gray card can test whether the reading measured by the camera's exposure meter is accurate. The following measurements must ensure that the exposure meter is accurate and the test is conducted under the same lighting conditions. Set the exposure meter to measure the reflected light (remove the white reflector). Exposure meter will be arbitrarily set a speed value, such as; 1/6th of a second. Then take a reading against the gray card surface and record it.

set the speed value of the camera to 1/6th of a second and take readings on the gray surface of the gray card. If the aperture value measured by the camera is the same as that of 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 of exposure meter cannot exceed 1/3 aperture.

Introduction of existing products in the market

At present, there are two kinds of most common gray cards on the market: one is the Kodak gray card imported from the original factory. A set of standard Kodak grey cards costs around 18 yuan, including two 8*1-inch grey cards, one 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.

There is another kind of domestic Minon grey card, including an 8*1 inch grey card and a 4*5 inch grey card. The market price is one set in 68 yuan. It is worth mentioning that, after research and testing by Beijing Meinong and 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, the Minon grey card adopts this reflectivity configuration.

one of these two kinds of grey cards strictly adheres to the traditional concept, while the other is innovative. Kodak gray card can give a small plastic cover, which is really a very thoughtful design. Meinong reduced an extra large gray card, lowered the cost, and promoted it to the market at the price of about 1/3 of the original product, which is undoubtedly the best choice for photographers who pay attention to cost performance.

Precautions for using grey card

Try to put the card close to the main body.

avoid the card surface reflecting light towards the camera.

For a plane subject (such as remake), the gray card should be parallel to the subject, and it is best to put it directly on the subject.

when shooting a stereoscopic object, face the gray card to the middle angle between the main light and the camera to avoid the card from reflecting light towards the camera.

point to the gray card along the optical axis of the lens to measure the reflected light.

if the subject is very light, subtract 1EV from the measured exposure reading.

if the subject is very dark, add 1EV according to the measured exposure reading.

if the main part is very light or dark, do not use this metering method. It is more appropriate to use multiple measuring points for reading.