This paper analyzes the focusing technology of digital camera shooting.

Automatic focusing mechanism of camera

In the process of shooting dynamic images, the distance between the camera and the object is constantly changing, so it often exceeds the depth of field, resulting in blurred images. In order to keep the image clear, we must constantly change the focus position of the lens to keep the image clear. This process of adjusting the focus position is called focusing or focusing.

The front end of the camera lens is specially equipped with a group of focusing lenses, including an outer lens and an inner lens, and the purpose of focusing is achieved by changing the position of the focusing lens. At present, there are many automatic focusing methods for camera lenses, which can be roughly divided into two categories: one is active focusing, including infrared and ultrasonic methods; The other is passive focusing, and the representative ones are the "solid triangulation" system developed by Canon and the recently popular TTL mode.

Usually, home cameras adopt active focusing. The principle is that when the lens is aimed at the target, a group of transmitters installed at the lower part of the camera lens emit infrared or ultrasonic waves, which are reflected by the photographed object and then received by the infrared sensor or ultrasonic sensor of the camera, so as to measure the distance and drive the focusing device of the camera to focus according to the measured distance. Its advantage is that it is not affected by lighting conditions and can work in a completely dark environment. But it can't work through the glass, and it can't work normally for objects that absorb infrared or ultrasonic waves and long-distance objects.

Most professional cameras use passive focusing. The principle of SST mode is that the light from the object passes through the fixed mirror and the movable mirror respectively, and then is reflected on two lenses and a prism respectively, and then is irradiated on the detection sensor and converted into an electrical signal. Then the microcomputer processor analyzes the obtained information, calculates the same distance as the photographed object, and controls the focusing motor of the lens to focus. The structure of TTL mode is relatively simple: the video signal is directly taken from the CCD sensor behind the camera lens, and then processed by the microcomputer, and the autofocus mechanism of the lens is adjusted according to these returned data. It has the advantages of correct long-distance focusing and ignoring the difference, but it can't work normally when the light is too dark and the contrast of the subject is low.

Camera focusing operation

Usually, ensuring the clarity of the shot picture is one of the most basic requirements for shooting, and focus adjustment is the most important part to ensure the clarity of the image. The process of camera focusing is the process of adjusting image definition.

In practice, the zoom lens is usually pushed to the wide-angle position (W) before focusing, because the depth of field is wide at this time, so it is easy to focus. We observe the clarity of the image through the viewfinder until we are satisfied. After focusing, push and pull the zoom lever to adjust the lens to the desired composition scene, and the focus will not change during zooming. However, it is difficult to focus with the telephoto position (T). Especially when shooting at close range, be sure to adjust the lens to the position with the largest focal length.

At present, all cameras have automatic focusing function, and the slightly advanced ones have manual focusing function. In the automatic state, it can basically meet the shooting in most environments. Unless you are an experienced photographer, it is better to rely on the camera automatic system first.

However, we know that the autofocus system is not omnipotent. All kinds of autofocus have their own characteristics and limitations, and manual focusing is needed in many cases. For example, in the following figure, when the main subject deviates from the center of the picture and is at the edge of the picture, the method of using manual focusing is as follows: first, switch the automatic focusing to manual, aim the subject at the center of the picture, adjust the definition to the best, then lock the focal length at a fixed position by using the locking function, and then re-compose and return to the original position.

The autofocus system is greatly influenced by the brightness of the subject. When the light is sufficient, the automatic aperture is narrowed, the depth of field is deepened, the focusing range is widened, and focusing is easy. In this case, when the subject moves or moves or translates, there will be no focusing failure image; However, when shooting a subject in dark light, it is difficult to focus because the aperture of the lens is wide open and the depth of field is shallow. The best solution is to increase the illumination of the subject.

Automatic focusing problem

The autofocus device of a camera is generally based on the center of the picture. Only a small area in the center of the picture is the detection range of autofocus. In this small area, the focus of the object can be automatically gathered, that is, if the subject is not in the center of the picture, autofocus will be biased. In addition, the autofocus system is greatly influenced by light, brightness, subject and other conditions, and there will be focusing deviation in some special cases, so it is best to use manual focusing in these occasions.

Auto-focus system for the following target or under the following shooting conditions, auto-focus devices often make wrong judgments. If autofocus is difficult, you need to focus manually.

A scene far from the center of the picture cannot get the correct focus. This is because the autofocus system is adjusted based on the center of the image.

One end of the subject is close to the camera and the other end is far away. A camera lens has a certain depth of field. For objects beyond their depth of field, the camera cannot focus on the objects in the foreground and background at the same time.

Shoot objects behind dirty, dusty or dripping glass. This is because it will focus on the glass. It will not focus on the object behind the glass. Please stick to the glass when shooting in front of the glass window.

Auto focus is also difficult to work when shooting objects behind fences, nets, rows of trees or columns.

Shooting objects in a dark environment. Because the light entering the lens is greatly reduced, the camera can't focus correctly.

Shoot objects with shiny surface, too strong reflection or too bright surroundings. Because the camera focuses on objects with smooth surface or high reflectivity, the subject will be blurred.

It is difficult to focus when shooting fast moving objects. Due to the mechanical movement inside the focusing lens, it is impossible to keep pace with the fast moving object. When the system tracks the shooting, it will make the scene fluctuate in out-of-focus and in-focus States.

Move the target behind the object. The autofocus system will mistake the moving object for the photographed object and focus on it.

Shoot targets with weak contrast or no vertical contour. Because the camera focus is based on the contrast object in the vertical direction of the image, such as a white wall, it may become blurred.

When it rains, snows or there is water on the ground, the autofocus system may not focus correctly.

If your camera focuses automatically by infrared or ultrasonic waves, it is difficult to focus when the subject can absorb infrared or ultrasonic waves; It is difficult to focus when the object is too far away for infrared rays or ultrasonic waves to reach the object.

Manual focusing is better than automatic focusing in 7 cases.

In most cases, autofocus undoubtedly has great benefits, but in some cases, manual focusing is a more sensible choice. In some cases listed below, autofocus is not always good. At this time, you should set the focus key to m, and then manually adjust the focus. Don't forget, switching manual focus can also play the role of focus lock. Sometimes, you can use this principle to focus on an object first and then make a composition. As long as you shoot an object at the same distance, you don't need to focus again.

1. Chaos scene

When the shooting scene is full of hundreds of elements-and each element has many details-manual focusing is essential. Especially when the object is small or has no specific shape, size, color or light and shade. Like a bunch of flowers, or a street full of pedestrians.

2. There are obstacles in front of the topic

If the subject is behind an obstacle and the obstacle is near the focus, autofocus can easily focus on the obstacle instead of the subject, and then manual focusing can show its talents. If you use a large aperture, you can also blur obstacles. For example, photographing animals in cages, people at windows or doors, and so on.

3. Buildings

Many architectural photographers, especially those who specialize in contemporary architecture, like to focus manually. The geometric shapes and lines of buildings often confuse the camera autofocus system, such as many skyscrapers.

4. High profile

When there is a bright area with strong contrast in the scene, it is also a test of autofocus. For example, shooting portraits in outdoor shadows, and there is a large bright light area next to the subject.

5. Low contrast

Low contrast scene is the nightmare of autofocus system. Low contrast means that the color and tone of the subject and background are close. One of the most common examples is the white snowman in the snow.

6. The background accounts for most of this painting.

The larger background relative to the subject will also make the autofocus system invalid. For example, if a short person is in front of a huge city street view, the automatic focusing system doesn't work, and switching to manual focusing can be done quickly and well.

7. Night view

When you want to shoot in a dark environment, you will find that manual focusing is your only choice. For example, when shooting the track of starlight, or when drawing light. In this case, focusing needs a lot of attempts, and the telephoto end of the zoom lens and LCD can be used to assist focusing.