Principle of photographic structure: perspective and depth

Lead: A pair of contradictions about vision is that when the image is projected on the retina according to the optical principle, the distant objects are smaller than the nearby objects, but the brain can understand their actual size when it gets enough hints.

Principle of photographic composition: perspective and depth are in the same field of vision, can the brain accept two realities at the same time? Objects in the distance are relatively small, their true size. The same thing happened in linear perspective. The two parallel sides of the road extend from us and converge in the distance, but at the same time we can feel that they are straight and parallel. The explanation for this phenomenon is called. Constant ratio? Or? The proportion remains the same? This is a perceptual mechanism that we don't know yet, which can solve the contradiction brought by the sense of depth. The influence of this on photography is that the recorded images are completely optical, so distant objects can only appear smaller, and parallel lines always converge. Photography, like painting, must adopt various strategies to strengthen or weaken the depth of the picture. The image has its own frame of reference, not the normal perception system.

The eternal relationship between the photographer and the real scene makes the sense of depth always important to the photos, which further affects the realism of the photos. The broadest perspective is the existence of objects in space and their relationship with the audience. In photography, perspective is more commonly called depth. We will describe the different types of perspective and other depth control methods later, and first consider how and why to use perspective. If we have the ability to influence perspective, under what conditions can it help photos strengthen or weaken the sense of depth? The enhancement of depth and strong perspective can enhance the audience's sense of existence in the actual scene, strengthen the representative characteristics of the subject and weaken the graphic structure.

The following table lists the main variables of various perspective types that affect the depth sense of photos. This variable is mainly determined by the situation and the influence exerted by the photographer.

Straight perspective view

In two-dimensional images, linear perspective is generally the most important perspective effect, which is characterized by converging straight lines. These straight lines are actually parallel lines in most scenes, such as the two sides of the road, the top and bottom of the wall, but once away from the camera, they seem to converge to one or more distant points. If they stay far enough in the picture, they will really meet at some point. If the camera is horizontal and the scene is landscape, then the horizontal line will converge to the horizon; If the camera is pointed upward, vertical lines like the edge of the building will converge to a certain point in the sky. Visually, it is difficult for most people to accept that this is a normal image.

The 20mm lens and viewpoint of linear perspective make the parallel lines of Hadrian Great Wall in northern England extend far beyond the camera position, which constitutes the classic condition of strong linear perspective. The height of the camera above the Great Wall determines the vertical distance that can be seen in the photo, and also determines the degree of convergence of lines.

In the process of convergence, all or most straight lines will become oblique lines, resulting in visual tension and sense of movement. The movement itself can enhance the sense of depth, and these oblique lines can guide the eyes in and out of the scene. In this way, all types of diagonal lines contain a sense of depth, including shadows that seem to have diagonal lines. Therefore, if the shadow produced by direct sunlight, especially the low sun in the sky, can strengthen the perspective. The viewpoint can determine the degree of convergence. The smaller the angle between the line of sight and the plane, the greater the degree of convergence? At least when the camera is close to the ground, the convergence effect will be very obvious and disappear at one point. The focal length of the lens is another important factor affecting linear perspective. When two shots are directly aimed at the scene position (according to experience, the object appearing in the lower part of the picture may be in the foreground) and overlap (if the outline of one object covers the other, it can be considered as being in front of the other). Finally, the wide-angle lens can contain more oblique lines in the foreground, thus dominating the structure of the image more obviously. Therefore, wide-angle lenses tend to enhance linear perspective, while telephoto lenses flatten it.

Contraction perspective

This is related to linear perspective, which is actually a form of it. Imagine a row of the same trees by the roadside. Looking along the road, you will see the familiar convergence effect, but for trees, they seem to be getting smaller and smaller. This is the so-called contraction perspective, which mainly appears on many identical or similar objects located at different distances. For similar reasons, familiar dimensions can be used as reference dimensions, which helps to establish a sense of perspective in the appropriate position of the scene. Related to shrinking perspective are position (according to experience, the object appearing in the lower part of the picture may be in the foreground) and overlap (if the outline of one object covers another object, it can be considered as being in front of another object).

Convergence and height

The angle between the line of sight and the plane of the convergence line determines the intensity of the perspective effect. If it is too low, it is difficult to see clearly, and if it is too high, the convergence effect is weak.

Convergence and focal length

Under the same viewpoint, wide-angle lenses with different focal lengths have the strongest convergence effect.

Spatial perspective

Atmospheric fog is like a filter, which reduces the contrast of distant parts of the scene and brightens the tone. Our familiarity with this effect (such as the endless horizon) makes our eyes take it as a deep hint. The fuzzy and foggy scene looks more profound than reality because of its strong sense of spatial perspective. This effect can be enhanced by using a backlight, as shown in the following example, and fog can be reduced without using a filter (a filter for filtering ultraviolet rays). The telephoto lens can show the spatial perspective effect better than the wide-angle lens when shooting various subjects, because there are fewer objects between the subject and the camera that are not affected by fog. When converting RGB color digital images into black and white by channel mixing method, adjusting the blue channel can also emphasize this effect.

The gradual change of light and dark tones shows the deepening effect of air fog, thus producing a sense of depth. The effect is strongest under strong backlight like this.

Shadow perspective

In addition to the brightening effect of fog on distant objects, bright objects appear in front and dark objects retreat. So the bright objects in front of the black background usually give people a prominent feeling and have a strong sense of depth. This effect can be controlled by carefully arranging the subject or carefully arranging the light. On the other hand, it will confuse the subject and the background.

Color perspective

Warm colors have the feeling of leaning forward, while cool colors retreat. Therefore, among other factors, for purely optical reasons, red or orange objects will have a sense of depth in front of the green or blue background. Similarly, proper positioning can be used to control the effect. The stronger the color, the stronger the effect. However, if the shades of colors are different, it is best to arrange them in the foreground.

This is a row of archways in Anhui, China. The eyes are assumed to be the same, getting smaller and smaller, resulting in a strong sense of depth. Because of the fog, there is also a spatial perspective effect in the picture.

The lack of spatial perspective and clear sky are two conditions that weaken spatial perspective. Just like this photo of the pier in the morning, the foreground and background are similar in tone, and the contrast is very small, so the sense of depth is very weak.

Clarity/clarity

Good clarity implies close range, and any clarity different from the foreground can enhance the sense of depth. Air fog has some such effects, and the most effective method is focusing. If the definition of different parts of the picture is different, will we take this as a depth hint from experience? Either the foreground is in Jiao Wai, or the background is in Jiao Wai, or both.