How to use grey card in photography photometry.

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The picture comes from Liquan

What is a grey card?

Grey card is the benchmark for accurately detecting 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 scene with complex light into 18% neutral gray. By recording the reflected light of the gray card with an exposure meter, you can get an accurate exposure value!

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.

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