What is autofocus (AF)? A detailed explanation of typical types and usage

Although autofocus (AF) is now a common feature in focusing in photography, many people may not be familiar with the detailed technology and its differences. This article will explain the basics of AF and its features. In this article, we will explain the basics and applications of AF in detail.

What is autofocus (AF)?

Autofocus (AF) refers to the ability of a camera to automatically focus on a subject; there are multiple methods of AF, and the range of focus (distance measurement point) can be set by the focus area.

The latest AF systems not only focus accurately, but also track moving subjects. Motion tracking performance is especially essential for fast-moving subjects such as sports and animal photography.

Difference from manual focus (MF)

Manual focus (MF), on the other hand, is a method in which the photographer manually turns the focus ring on the lens to adjust the focus, and while MF allows the photographer to adjust the focus more as intended, it also requires familiarity and experience.

It is important to note that AF is not always the best solution, and there are situations where it is difficult to focus with AF, depending on the subject and environment. For example, there are many situations in which MF is better suited for dark or low-contrast subjects or complicated compositions.

Therefore, by properly using both AF and MF depending on the situation, you can create beautiful photographs of any subject.

Types of AF operation

There are three main modes of operation for the AF function of a camera. These are generally referred to as AF-S, AF-C, and AF-A, and each is suitable for different shooting scenes and subjects. Selecting the appropriate mode accordingly will make it easier to focus more reliably.

AF-S(single AF)

AF-S is the best mode for shooting stationary subjects. In this mode, once the subject is in focus, it remains fixed, making it ideal for landscapes, portraits, and other situations where there is little movement in the subject. Normally, the shutter release button is pressed halfway to bring the subject into focus, and then the composition is determined before shooting. This allows the photographer to take a picture in which the subject is firmly in focus.

AF-C (Continuous AF)

AF-C is a mode that keeps tracking a moving subject. This mode is suitable for situations where the subject is constantly moving, such as sports or wildlife photography. Normally, in this mode, the camera will constantly focus on the subject for as long as the shutter release button is pressed halfway down, and then the shutter is pressed at the moment the picture is taken. Therefore, even with fast-moving subjects, the camera can take pictures with minimal subject blur without missing the shutter release opportunity.

AF-A (AF Switching)

AF-A is a mode in which the camera body automatically detects the subject's movement and switches between AF-S and AF-C. This mode is suitable when the subject moves or stops and it is difficult to predict the next movement. For example, it is easy to use for subjects that suddenly start or stop moving, such as children or pets.

Types of focus areas

To use the camera's AF function more effectively, it is also important to set the focus area. The focus area refers to the area that the camera will focus on. The types and names of available focus areas vary depending on the camera model, but in general they are as follows

Wide focus

Wide focus is a mode that automatically focuses the camera based on the entire frame. This mode is suitable for shooting subjects that move unexpectedly, such as animals. The advantage is that the camera automatically selects the focus position, allowing the photographer to concentrate on the subject and composition. However, care must be taken because the camera may focus on an unintended position.

Tracking Mode

Tracking mode is used in combination with AF-C (Continuous AF). In this mode, the camera continues to track the focus on the selected subject within the focus area. It is very effective for shooting subjects that are moving rapidly or moving within the frame. For example, it is suitable for chasing a specific person at a sports game or athletic meet, or for photographing animals such as wild birds.

Zone focus

Zone focus is a mode that automatically focuses within a specified area. This mode is especially useful when a moving subject moves within a specific range. For example, it is useful in sports or bird photography when the subject's movement is somewhat predictable. The camera focuses on the nearest subject or the area of highest contrast within the specified zone. This makes it more difficult for the camera to lose focus even if the subject moves slightly.

Center

Center Focus is a mode that automatically focuses on the area near the center of the frame. This mode is suitable for portraits and other situations where the subject is centered on the screen. If the subject is not in the center, you can use the Focus Lock function to lock the focus on the subject to change the composition.

Spot (Flexible Spot) and Extended Spot

Spot focus (also called Flexible Spot) is a mode that allows you to pinpoint focus on a specific small area. Generally, the size of the spot can be adjusted in this mode, and the appropriate size can be selected based on the size of the subject. This mode is ideal when precise focusing is required or when shooting complex compositions with many subjects.

Extended Spot is a mode that extends the function of Spot Focus. When it is difficult to focus on a spot, the focus area is expanded around the spot to assist in focusing. This allows for more reliable focusing and is useful for shooting moving or low-contrast subjects.

What is a distance measurement point

When setting the focus area, the term “distance measurement point” (also called the AF frame) is used. The distance measurement points are the focus detection points that the camera uses to autofocus. These points are usually visible in the viewfinder or on the LCD screen. The number of focusing points varies from camera model to camera model. In general, the more focusing points a camera has, the more flexible and accurate focusing is possible.

In particular, high-end models tend to have more distance measurement points, making it easier to focus on the smallest details of the subject and on locations near the edges of the frame. However, it is important to select the appropriate range and number of distance measurement points to suit the shooting scene and your preferences, rather than simply having a large number of distance measurement points itself.

Autofocus (AF) mechanism

There are a variety of AF technologies and methods, but currently there are three main methods, and a combination of these methods is used to achieve highly accurate AF. Here, we explain the mechanism and features of each AF method.

Phase detection AF

Phase detection AF is a method that divides the light incident on the lens into two parts and then uses a dedicated AF sensor to measure the displacement of the image formation position and bring it into focus. The advantage of this method is that it can focus more quickly than other methods. It is suitable for sports and fast-moving subjects, and allows photographers to capture fleeting shutter opportunities.

However, phase detection AF has several limitations. First, it requires a mirror and mechanism dedicated to AF, so it is basically a method used in single-lens reflex cameras, which also require a larger camera body.

Image phase detection AF

Image phase detection AF, like phase detection AF, detects the misalignment of the image formation positions of two images to bring them into focus. However, since this method incorporates an AF sensor on the image sensor to detect the phase difference, it has the advantage of being able to be mounted on mirrorless SLR cameras. It is faster than contrast AF, which will be discussed later, and can also be used for AF during video recording because it is reflected in the live view. It also contributes to making mirrorless SLR cameras smaller and lighter.

On the other hand, the disadvantages are that it tends to be somewhat slower than phase detection AF with respect to AF operation, and it tends to consume more power. In addition, in general, AF operation in dark areas tends to be inferior to contrast AF.

Contrast AF

Contrast AF is a method of focusing so that the contrast of the image on the image sensor is greatest while the lens is moved back and forth. This method is characterized by its relatively high AF accuracy, especially for stationary subjects in dark areas. However, because of its principle, it is necessary to check the contrast while moving the lens, which has the disadvantage that the AF tends to be slow.

Contrast AF does not require an AF sensor, so it can be used for live view and is also used in compact digital cameras and smartphones. In addition, since focusing is performed directly on the image sensor, AF is more accurate in principle.

Thus, since each method has its own advantages and disadvantages, combining these methods results in more accurate and faster AF.

Tamron's autofocus (AF) technology

Tamron incorporates the VXD (Voice-coil eXtreme-torque Drive) in its lenses as the actuator that supports the latest AF technology.

VXD enables comfortable focusing from minimum focusing distance to infinity, even with large aperture lenses, and demonstrates stable AF performance over a wide shooting range.

The VXD's extremely high focus tracking performance also makes it ideal for shooting fast-moving subjects. For example, it will not miss a moment in sports photography or portrait photography.

Another important feature of the VXD is that it generates little vibration during AF drive. This is a great advantage when shooting in environments where quietness is required. For example, it is ideal for shooting in quiet situations such as ceremonies and concerts, as well as for movie shooting.

Use your knowledge of AF for lens selection and focusing

Understanding how AF works, its types, and how to select the focus area will help you make the appropriate settings for various shooting situations. On the other hand, while AF is a useful feature, there are times when MF is more optimal, and understanding the characteristics of AF and using it appropriately will allow you to take pictures that are more in line with your image.