Please use your knowledge of photography to explain to me what image contrast is and what scene contrast is.

The ratio of light and dark in two different parts of a photographic frame; the ratio of the brightness of a brighter part to a smaller part of a scene or image. The human eye has a wide adaptability to changes in brightness, so the judgment of the absolute value of scene brightness is not sensitive enough. However, the human eye is very sensitive to coexisting brightness differences in the scene. This sensitivity is very little affected by its absolute value within a certain brightness range. Therefore, it is of practical significance to use the brightness ratio or the difference in logarithmic values ??of brightness to express the contrast degree of a scene or image.

In the process of photography and film production, the contrast of the image depends not only on the contrast of the scene being photographed and the change in brightness ratio of the scene after the optical image is formed by the camera, but also to a large extent. Depends on the contrast properties of the photosensitive film used and the degree of development specified for this purpose. Therefore, in order to obtain an image with moderate contrast, it is necessary to test and select the contrast characteristics of the film under specific processing conditions. During the film processing process, the contrast changes of the image are controlled by the contrast coefficient. Contrast coefficient refers to the ratio of the density difference between two points in the straight line part of the film's photosensitivity characteristic curve (see photosensitivity measurement) to the corresponding logarithmic difference in exposure.