How to understand the contrast in photography

How to understand the contrast in photography

What determines the contrast? The first is the contrast of the scene, that is, the actual reflection value of the scene surface. If a subject is properly exposed and the tone of the subject ranges from highlights to dark shadows, a negative of Gao Fancha is obtained. For example, shooting such a scene with a wide snow surface, the skier slides in a black snow coat, and the sun shines on the snow surface behind him, which will produce a contrast negative.

On the contrary, a normal exposure film shoots a lighthouse, and the lighthouse is in a foggy environment, so this scene will produce a very flat and low contrast film, if this film is developed normally.

Comprehensive lighthouse photos. The second factor of comparison is exposure. If the film is overexposed, the contrast will decrease, even if the development is normal, the contrast will decrease. In order to understand how this happens, we should know the photosensitive characteristic curve of film in photography, that is, H? D curve. It is the basic tool of photography theory. This curve is extended by the names of two scientists: Haider and Dreyfield. This is a method to express photosensitivity developed in 1890. This curve is called the photosensitive curve. (This curve is also called: D? Me? OGE curve), which shows that the relationship between silver accumulation and exposure has developed to a certain extent. It can be seen that the greater the exposure, the more silver accumulation until a certain point is reached.

At the toe of this curve, every increase in exposure only slightly increases the density. After reaching the shoulder, each increase in exposure will not increase the density proportionally. Only between the toes and shoulders, in the straight part of the curve, whenever the exposure increases a little, the density always increases proportionally.

What does this have to do with filming and developing? When the film is exposed, if it is overexposed, the highlight tone will move to the shoulder, while the middle tone is still in a straight line, resulting in a negative with a small contrast between the middle tone and the highlight tone, so overexposure will reduce the contrast.

On the other hand, if the film is underexposed, the tone of the highlight part will fall to the straight line of the characteristic curve, while the tone of the middle tone or shadow part will still be at the toe. In this way, the tone density of high light will increase faster than that of middle tone or shadow part, which will produce a negative with great contrast between high light tone and middle tone or shadow part, so it can be said that insufficient exposure can increase the contrast;

Because film can provide greater exposure latitude for overexposure, that is, it can provide greater exposure latitude than under exposure, so overexposure is better. If it is an ordinary scene, overexposure can have five or more exposure tolerances on most negative films without losing the details of highlights, but it cannot be underexposed. If an ordinary scene is exposed to two levels, the details of the shadow will be completely lost. When overexposed, the contrast will decrease unless the overexposure is serious.

Contrast and development, photosensitive characteristic curve, is H? D curve can show the relationship between density and exposure and development. Now let's see how development affects density and contrast. There are two main points:

1. Increasing film development will increase the contrast;

2. Reduce contrast by reducing development.

How to increase or decrease the degree of development? Generally speaking, increasing development time will increase the degree of development. Another method is to increase the temperature of the developer, which can also improve the degree of development, because the higher the temperature, the greater the energy of the developer. The third way is to change to a developer with strong vitality.

To understand how development affects contrast, a simplified model can be used. Let's assume that there are ten layers of silver halide particles on the film irradiated by light. This is a simplified model. In reality, this is not the case, but there are many, I am afraid, thousands of layers thick and millions of layers thick in some places. Now suppose that one layer of silver halide particles appears every minute, five layers appear every five minutes, and 10 layer appears every minute.

At the end of the first minute, the first layer is developed, which includes each part of the light receiving area of the film. If development is stopped, the only contrast is due to the difference in silver accumulation between the part that has not been irradiated with light and the part that has been irradiated with any light. At the end of three minutes, two or three layers of silver shadows appeared in the least exposed part of the shadow, that is, the development exposure produced silver shadows with different thicknesses, but the part with strong highlights was far from being displayed, because it only reached the third layer, although it had been affected by the exposure to the tenth layer, but at this point of three minutes, there was only a small contrast on the negative.

Continue to develop for a longer time and increase the contrast between these regions, that is, some regions that are close to full development and some regions that are far from full development. Therefore, increasing development will increase contrast, and reducing development will also reduce contrast.

That is to say, the higher the degree of development, the greater the density difference, that is, the greater the contrast when the same exposure is increased. Sometimes you will see the word GAMMA with negative film mentioned in some technical classes. γ refers to the contrast on the negative film, specifically, it refers to the difference between the density of the thickest part and the density of the smallest and thinnest part on the negative film, which can be expressed by the inclination of the straight part of the photosensitive characteristic curve. Most of the negatives to be enlarged are developed until the gamma is equal to? 6 or to? 70 。

The above is what Bian Xiao shared for you about how to understand the contrast in photography. For more information, you can pay attention to Global Ivy and share more dry goods.