Explanation of depth of field in photography?

-The 15th digital photography guide for beginners.

Watch Yun Tao write/photograph/draw

Some of the photos we usually see are clear in the whole picture, and some are only part of the scenery. In some photos, the people in front are very clear, and the people in the distance are very vague. this is

Camera: Nikon D300S Lens: Nikon 70-200/2.8

Aperture: f2.8 shutter: 1/6400 seconds aperture is preferred.

How did it happen? What is the reason for this phenomenon in photography? This problem is a big problem for beginners to learn photography creation. So I study and discuss this phenomenon with my friends who have just come into contact with photography in order to learn the basic knowledge of depth of field in photography.

1, what is the depth of field?

This problem can be explained by our eyes. When we look at an object, in any case, you can't see all the near and far scenes clearly. The human eye can quickly observe distant and nearby scenes by zooming. When a camera lens takes pictures, each picture will only have one focal length. How can we make clear images of different scenes from far and near on the negative? This is a problem that the principle of depth of field can guide.

When we take pictures, we focus through the expansion and contraction of the lens. If we take a photo from the right front of a row of people, we focus on the fourth person. It is not only the fourth person that is clear in the photo, but also the second person, the third person and the fifth, sixth, seventh, eighth and ninth people, as shown below:

The above figure shows the actual clearance range after focusing on a certain target.

As can be seen from the picture, the characters after the first person and the tenth person are gradually blurred. This practical example tells us that not only the focused scenery is clear, but also the scenery with a certain distance before and after focusing can form a clear image on the photosensitive film or device. The distance between the front and back ranges of a clear image is what we call the depth of field.

By mastering the principle of depth of field, the clear range of photos can be controlled, so that the virtual and the real can coexist and change, and the purpose of artistic creation can be achieved. As we can see from the above figure, the depth of field is divided into two parts: the part before the focus target is called the foreground depth of field, and the part after the focus target is called the back depth of field. The sum of the front and back depth of field, that is, from the nearest clear point to the farthest clear point (the second person to the ninth person in the picture) is Quan Jingshen.

2. Factors affecting the depth of field

The main factors affecting the depth of field are as follows:

(1) Aperture size: the aperture is a light hole, and the larger the aperture, the larger the light hole; The smaller the aperture, the smaller the aperture. Under the condition of constant object distance and focal length, the smaller the aperture, the greater the focal depth, the greater the focal depth and the greater the depth of field. This problem involves the principle of optics. We know that the aperture is large, the depth of field is small, the aperture is small, and the depth of field is large. As shown in the figure:

⑵ focal length: the principle of lens dispersion circle tells us that the longer the focal length of the lens, the shorter the depth of field under the same aperture coefficient and object distance; The shorter the focal length of the lens, the longer the depth of field. As shown in the figure below:

⑶ Object distance (distance): The depth of field is directly related to the object distance, which is determined by the * * * yoke principle of object distance and image distance. The conclusion of this principle is: when the focal length and aperture of the lens are exactly the same, the closer the object distance, the shorter the depth of field; The farther the object distance, the longer the depth of field. As shown in the figure below:

Use a wide-angle lens to increase the depth of field, so that the foreground, middle and distant view are clear.

3. The main application of depth of field in photography

(1) indicates the depth of the scene.

In a picture, it is relatively easy to show the height and height of the subject, and methods such as increasing the object distance and reducing the focal length can be adopted. However, all scenes have three-dimensional space. In order to express the three-dimensional space or shape of the scene in photographic modeling, that is to say, to express the depth and length of the scene, the principle of depth of field must be applied.

For example, a building can be photographed from the front, but it lacks three-dimensional sense. If you shoot from the side, you can show the three-dimensional space and its three-dimensional shape of the building by applying the principle of depth of field. In other words, when we want to express the depth length of the scene, we must use a large depth of field.

Cologne cathedral, Germany

The picture above is my picture of Cologne Cathedral. The front lens only shows the height of the front of the church, but it lacks three-dimensional sense and basically has no sense of depth.

The picture below shows the rural scenery I took in France, with a large depth of field, showing the depth of the scene.

French rural scenery

The next photo is on the outskirts of London. The red car is both the main body and the foreground, with deep paths and trees in the distance. Showing deep affection for the scenery. In other words, it is an application of depth of field in photographic modeling to express the depth of the scene with greater depth of field.

In the suburbs of London, England

(2) Highlight the theme.

When we shoot some scenes and need to highlight the main part, we must use short depth of field. For example, when depicting someone's mental outlook and psychological activities, close-ups of people should be taken with short depth of field. The result of highlighting the subject is to put the background and sometimes the foreground outside the clear range and become blurred.

Aperture: f6.3 shutter: 1/800 seconds lens focal length: 750mm.

Use a long lens and a large aperture to make the depth of field short and shallow to blur the background.

Especially when the background is chaotic, the background should be blurred. Sometimes the subject is clear and the background is vague, which can also give people a sense of depth in space. This sense of depth is different from a large depth of field. The former is solid on the front and empty on the back, while the latter is distinct.