What does autofocus mean? How to set autofocus?

Understanding the working principle of the autofocus system in the camera will ensure that you get a clear image.

1. When focusing in a low contrast area, autofocus will not work.

If you use a camera to focus on a white wall without texture or a blue sky without clouds, the camera's autofocus system will try to focus several times, because there is no contrast and transition in the blank white wall, so you can use the camera to judge the focusing accuracy.

However, if you find a contrast area on the wall and focus on the middle part, then the autofocus system can complete the work, and the higher the contrast, the better!

2. Active and passive autofocus

There are two kinds of automatic focusing (AF) systems-active and passive. The working principle of active autofocus is to aim the red beam at the object and then reflect the light back to the camera to determine the distance between the camera and the object. After the camera knows the value of the distance, it instructs the lens to adjust the focal length according to this information. Passive autofocus systems work in very different ways. It uses "phase detection" or "contrast detection" or a combination of both to determine the contrast area.

3. Phase detection autofocus, contrast detection autofocus and hybrid autofocus.

A microlens array is used to operate the phase detection autofocus. When light passes through them, it will split into a pair of images. Then measure the distance between these images to see how far the focus of the subject is forward or backward. Then, the camera can use this information to send accurate focus correction instructions to the lens. This autofocus is very fast and suitable for shooting fast-moving subjects.

Contrast detection autofocus is based on software algorithm, which can "check" the edge details of the image area. It is basically the part of the scene that needs to be focused on. It is scanned by the camera and uses the lens to quickly switch the focus from the foreground to the background until the object is completely clear. This is a slow mode.

Hybrid autofocus mode means that the camera can easily switch between phase detection autofocus and contrast detection autofocus, so as to use both at the same time under different conditions.

4. Focus

The focus is the small blank square or point you see through the viewfinder. Entry-level cameras usually have a simple multi-focus autofocus system, while professional cameras have a complex multi-focus autofocus system.

For example, Nikon D3500 has 1 1 autofocus points, while Nikon D8 10 is equipped with 5 1 autofocus points. However, it is important to pay attention to not only the number of coordination centers, but also different types of coordination centers.

Nikon D3S @ 500 mm, ISO 1600, 1/800, f/8.0.

The more cross sensors there are in the camera, the better and more accurate the phase detection autofocus is. That's why when the new camera was announced, it reported "X focal points, X cross sensors".

5. Other factors affecting autofocus performance

The light quality is another important factor, because it is difficult for the camera to determine the contrast in low light conditions, and passive autofocus depends entirely on the light passing through the lens. If the quality of this lamp is poor, the autofocus is also poor.

The focus detection range is also very important, because it will affect the sensitivity of the camera's autofocus system to weak light.

Regarding the maximum aperture of the lens, ideally, the aperture of the lens should be between f/2.0 and f/2.8 to achieve the best autofocus performance. A small maximum aperture value, such as f/5.6, means that there is less light passing through the lens and autofocus is difficult to operate.

6, autofocus mode

Single autofocus mode (AF-S)

Single autofocus (AF-S), also known as Nikon's single area autofocus and Canon's single autofocus, is a simple focusing method. Choose a focus, and the camera will only look for contrast at this focus. When the focus key is pressed, the camera will focus once, and if the subject moves, it will not "interrupt" the focus. Focus remains "locked".

Nikon D700 @ 420, ISO 400,11000, f/5.6

Continuous autofocus (AF-C)

Another focusing mode is called continuous autofocus (AF-C), which is also called AI servo in Canon world. This mode is used to track moving objects. The biggest advantage of AF-C mode is that if the subject moves, it will automatically refocus.

Nikon D3S @ 300, ISO 3200, 1/250, f/8.0

7. autofocus with af-a/ mixed mode

A mode called AF-A or AI Focus AF (Canon) is essentially a mixed mode, which can automatically switch between AF-S and AF-C modes. If the camera thinks the subject is still, it will switch to AF-S, and if the subject is moving, it will automatically switch to AF-C mode.

Continuous servo (AF-F) focusing mode

Nikon has introduced continuous servo autofocus, also known as AF-F, which is specially used to record video through its DSLR and mirrorless camera.

Nikon D700 @ 550, ISO 800,11250, f/5.6

Auto focus area mode

Single point autofocus mode

Single-point autofocus is a special mode of Nikon. It uses contrast detection autofocus to focus accurately on a small scene, and the autofocus point becomes a small point, which can be slowly moved to any part of the screen, including the outer edge, only in AF-S mode.

Single-point autofocus area mode

Here, the camera only uses one focus point, and you can choose to get the focus point in the viewfinder. Therefore, if the focus moves up/down/left/right, the camera will only detect the contrast at this specific focus and use this mode when shooting landscapes and other still objects.

Dynamic AF region

In this mode, a focus will be selected and the camera will focus on this specific focus first. But after getting the focus, if the subject is moving, the camera will use the surrounding focus to track the movement of the subject and keep the focus on the subject. You should track the subject by translating the camera with the subject, and make sure that the subject stays near the original selected focus. If the camera selects surrounding/different autofocus points, it may not be directly visible in the viewfinder during shooting.

Automatic area autofocus

This is a method of focusing. According to what you shoot, it will automatically choose the object to focus on. This is a very difficult mode because it actually detects and automatically focuses on the skin color of the person in the picture.