Generation of color cast

In the process of shooting, due to light or angle problems, the photos taken may have color cast.

For example, in a snapshot, the reflected light will illuminate the scene. Or, incorrect development, printing, scanning, etc. , will make the image color cast. The reasons are generally the following main factors:

Improper exposure of luminometer.

The color of everything in the world is presented by the reflection of light by matter. In the study of brightness, it is found that the reflectivity of colorless snow is about 98%, and that of carbon black is about 2%. 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.

However, the luminance meter measures not only the illuminance, but also the reflectivity of the object. Under the same lighting conditions, the readings of carbon black and snow are quite different, so snow is not white and carbon black is not black.

Second, the environmental color

The color of the surrounding environment is reflected on the subject, which is called environmental color. For example, taking a commemorative photo, the subject's face will turn green when standing in the middle of green leaves, and turn red when standing in the middle of red flowers. Obviously, in actual shooting, the ambient color can easily cause the color cast of the subject. The color deviation of the main color caused by the ambient color is difficult to correct during color amplification or color amplification. In order to reduce the influence of ambient color, try to avoid the subject when shooting (except rendering the subject with ambient color).

Third, the camera color temperature does not match the illumination color temperature.

The color of the object will change due to the color of the projected light, and the photos taken under different light conditions will have different color temperatures. Because CCD can't automatically correct the change of light like human eyes. At this time, it is necessary to set the white balance artificially to restore the color. If the white balance is not set properly, the picture will be seriously color cast.

Four, the main color errors in color expansion

Because most of the current color enlargers are designed for mass production, the color correction of negative film is automatically carried out according to LATD. If the main body on the negative film accounts for a small proportion in the picture, but the color of the whole picture is rich and single, then the main body will appear color cast when the color is enlarged, which is called "color error of the main body of the picture". For example, if you shoot a white ceramic handicraft on four curtains of red, green, blue and black, a printed color photo will appear: the handicraft on the red cloth is blue; Handicrafts on green cloth are magenta; Handicrafts on the blue cloth are yellow; Handicrafts placed on black cloth are not color-cast, which is the result of automatic color correction by LATD. In actual photography, we often encounter the above-mentioned "color error of the main body of the picture", such as taking a color certificate photo. If red cloth is used as the background, the subject's face will appear "pale" (actually blue); If you choose blue cloth as the background, the subject's face will be yellow. Use achromatic colors (black, white, gray, etc. ) as the background, the main body is not color cast.