What is the Auto Focus and how to realize by VCM? - Supertek

29 Apr.,2024

 

What is the Auto Focus and how to realize by VCM? - Supertek

Auto Focus is to move the position of the entire lens by a small distance, control the focal length of the lens, and achieve a clear image, it is a commonly used method in a mobile phone cameras, and the autofocus is achieved by the working of VCM. The technology behind autofocus has evolved over the years, and one of the most popular methods of achieving autofocus in cameras is through the use of Voice Coil Motor (VCM) technology. In this article, we’ll take a closer look at autofocus and VCM, how they work together to achieve quick and accurate focusing, and their advantages and limitations.

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Introduction

Autofocus is a technology that allows cameras to automatically adjust the focus on a subject or scene, rather than requiring the user to manually adjust the focus. Autofocus is particularly useful in situations where the subject is moving or where the distance between the camera and subject is changing. By automating the focus, photographers can capture sharp images quickly and efficiently.

One of the key components of autofocus is the use of motors to adjust the position of the lens elements. There are several types of motors used in cameras, including stepper motors, ultrasonic motors, and Voice Coil Motors (VCMs). Of these, VCMs are becoming increasingly popular due to their speed, accuracy, and low power consumption.

II. What is Autofocus

Autofocus, commonly abbreviated as AF, is a feature that can be found in cameras and enables them to automatically adjust the focus of the lens to ensure that the subject is sharp and in focus. This feature has significantly transformed photography and has made it more accessible to both amateur and professional photographers.

Prior to the invention of autofocus, photographers had to focus their lenses manually, which was a tedious and time-consuming process that often resulted in blurry images, particularly when capturing moving subjects. However, with the introduction of autofocus, the camera takes care of focusing the lens, making it easier for photographers to capture clear and sharp images.

 

How does Autofocus Work?

Autofocus is a feature commonly found in cameras that allows them to automatically adjust the focus of the lens to ensure that the subject is sharp and in focus. The autofocus mechanism in a camera works by measuring the distance between the camera and the subject and adjusting the position of the lens elements until the subject is in focus. There are several types of autofocus mechanisms, including contrast detection, phase detection, and hybrid autofocus.

  • Contrast detection autofocus works by analyzing the contrast in the image and adjusting the focus until the contrast is maximized. This method is commonly used in mirrorless cameras and point-and-shoot cameras.
  • Phase detection autofocus, on the other hand, works by splitting the incoming light into two images and comparing the phase difference between them. This method is commonly used in DSLR cameras and provides fast and accurate focusing.
  • Hybrid autofocus combines both contrast detection and phase detection autofocus to achieve faster and more accurate focusing. This method is commonly used in mirrorless cameras.

The VCM of the mobile phone camera needs the driver IC to complete the focusing. The driver IC controls the power supply current of the VCM to determine the moving distance of the lens mounted on the VCM, so as to adjust to the appropriate position to capture a clear image.

 

After entering the auto-focusing mode, the Driver moves from 0 to the maximum value, so that the lens moves from the original position to the maximum displacement. At this time, the sensor imaging surface automatically captures pictures and saves them in the DSP. The DSP calculates each picture through these pictures. MTF (Modulation transfer function) value, so as to find the maximum value in this MTF curve, and through the algorithm, get the current size corresponding to this point, once again instruct the Driver to provide this current to the voice coil, and stabilize the lens on this imaging surface , so that automatic focus is achieved.

III. VCM Technology

Voice Coil Motor (VCM) is a type of motor that uses a magnetic field to move a coil back and forth. In a camera lens, the VCM is used to adjust the position of the lens elements, which changes the focus of the lens. Compared to other motor types, VCMs are faster, more precise, and consume less power.

The VCM system in a camera lens consists of two main components: the voice coil and the permanent magnet. Voice Coil Montor working principle is that in a permanent magnetic field, the stretching position of the spring is controlled by changing the DC current of the coil in the motor, thereby driving the lens up and down movement.The voice coil is attached to the lens elements, and the permanent magnet is fixed in place inside the lens barrel. When a current is passed through the voice coil, it generates a magnetic field that interacts with the magnetic field of the permanent magnet, causing the coil to move back and forth. This movement of the voice coil changes the position of the lens elements, which changes the focus of the lens.

One of the advantages of VCM technology is its ability to move the lens elements quickly and accurately. This allows the camera to focus on the subject in a fraction of a second, making it easier for photographers to capture sharp images. VCM technology is also quieter than other motor types, which is especially important for photographers who work in quiet environments.

VCM Classification

Voice Coil Motors (VCMs) are classified based on their construction and design. Here are some of the commonly used classifications for VCMs:

  1. Moving Magnet Type VCM: In this type of VCM, the permanent magnet is stationary, and the coil moves to change the position of the lens elements. This design is commonly used in autofocus lenses for digital cameras.
  2. Moving Coil Type VCM: In this type of VCM, the coil is stationary, and the permanent magnet moves to change the position of the lens elements. This design is commonly used in actuators for hard disk drives and other industrial applications.
  3. Flat Type VCM: In this type of VCM, the coil and magnet are both flat and are arranged parallel to each other. This design is used in applications where space is limited, such as in mobile phones and other portable devices.
  4. Cylindrical Type VCM: In this type of VCM, the coil and magnet are cylindrical in shape and are arranged concentrically. This design is commonly used in autofocus lenses for digital cameras.
  5. Linear Type VCM: In this type of VCM, the coil and magnet are arranged in a straight line, and the movement of the coil is linear. This design is commonly used in actuators for linear motion control.
  6. Rotary Type VCM: In this type of VCM, the coil and magnet are arranged in a circular pattern, and the movement of the coil is rotational. This design is commonly used in actuators for rotary motion control.

IV. Realizing Autofocus with VCM

To achieve autofocus using VCM technology, the camera sends a signal to the lens to move the lens elements until the subject is in focus. The camera measures the distance to the subject using various sensors, such as contrast detection or phase detection, and uses this information to adjust the position of the lens elements. The VCM is used to move the lens elements back and forth until the subject is in focus.

The process of achieving autofocus using VCM technology happens very quickly, with most cameras achieving focus in less than a second. This is because VCMs are very responsive and can move the lens elements quickly and accurately.

Mobile phone cameras widely use VCM to achieve auto-focus function. Through VCM, the position of the lens can be adjusted to present a clear image.

V. Advantages and Limitations of VCM-based Autofocus

VCM-based autofocus has several advantages over other types of autofocus. One of the main advantages is its speed and accuracy. VCMs can move the lens elements quickly and precisely, allowing the camera to focus on the subject in a fraction of a second. This is especially useful for capturing fast-moving subjects or for taking photos in low light conditions.

Another advantage of VCM-based autofocus is its low power consumption. VCMs consume less power than other types of motors, which means that cameras with VCM-based autofocus can last longer on a single battery charge.

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However, VCM-based autofocus also has some limitations. One of the main limitations is the noise generated by the VCM when it is in operation. Although VCMs are quieter than other motor types, they can still generate a noticeable amount of noise, especially in quiet environments. This can be a problem for photographers who need to work silently.

Another limitation of VCM-based autofocus is its ability to focus on certain types of subjects. Because VCMs rely on contrast or phase detection sensors to measure the distance to the subject, they may have difficulty focusing on subjects with low contrast or in low light conditions. In these situations, photographers may need to switch to manual focus or use a different autofocus mechanism.

VI. Conclusion

In conclusion, autofocus is an essential feature in modern cameras that allows to capture sharp images quickly and efficiently. Voice Coil Motor (VCM) technology is one of the most popular methods of achieving autofocus in cameras due to its speed, accuracy, and low power consumption.

The VCM system in a camera lens consists of a voice coil and a permanent magnet, which work together to adjust the position of the lens elements and achieve focus on the subject. VCM-based autofocus has several advantages, including fast and accurate focusing and low power consumption. However, it also has some limitations, such as noise generation and difficulty focusing on certain types of subjects.

Overall, VCM-based autofocus is a powerful tool for photographers that can help them capture sharp images quickly and efficiently. As camera technology continues to evolve, we can expect to see further improvements in autofocus and VCM technology.

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Autofocus (AF) Definition - What is ...

Technical explanation of Autofocus

Autofocus (AF) technology first came into being in very crude forms rather slowly during the 1960’s and 1970’s, and was only widely adopted by major camera companies in the 1980’s. Today, most consumer and professionally oriented lenses and cameras offer autofocus, with only a few speciality lens companies offering lenses that must always be focused manually.

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Different types of autofocus technologies are used today, and the AF system included in your camera will depend on what type of camera system it is. Generally speaking, most autofocus systems fall into two different categories: autofocus that is performed by the image sensor itself, and focusing that is done by a separate AF module.

Optical Phase-Detect Autofocus in SLR Cameras

If a camera has a mirror and a shutter, it will use an autofocus technology known as phase-detect autofocus. This type of AF system usually relies on an AF module which is separate from the image sensor itself. This AF module focuses the lens by using light that is bounced onto it from another mirror hidden behind the main one.

This AF technology has become highly accurate, and high-end professional cameras have impressive, intelligent phase-detect AF systems. However using a mirror and a separate autofocus module has one inherent flaw: it isn’t actually using the sensor itself to verify focus.

To compensate for minuscule (but consistent) discrepancies in a camera or lens’ autofocus precision, many modern cameras offer an autofocus fine-tuning system. (See AF Microadjustment)

Sensor-Based Autofocus in Mirrorless Cameras

Cameras that don’t have a mirror (also, SLR cameras that have a “live view” mode) use the image sensor itself to perform autofocus. This is usually performed by a method known as contrast-detect AF, which was previously inferior at both low-light autofocus performance, and continuous subject tracking.

However, some recent mirrorless cameras (and live view DSLRs) have developed a method of using the image sensor itself to perform phase-detect autofocus. Other cameras have gained a hybrid autofocus system, that performs both phase-detect and contrast-detect autofocus on the image sensor.

Both of these newest autofocus systems can be extremely accurate and less prone to errors, especially in optimal (bright) lighting conditions. The newest iterations of these technologies are finally rivaling the autofocus performance of even the best traditional optical phase-detect AF systems.

Other Autofocus Related Terms And Definitions

Autofocus systems have numerous features and options, some of which are sometimes confusingly named. Here is a brief list of some of the common terms used in Autofocus systems:

  • Focus Points: The individual points around the viewfinder where autofocus is actually performed. Most cameras allow you to either select an individual focus point and move it around the viewfinder, or let the camera automatically pick whichever focus point it thinks is best.
  • Cross-Type Focus Points: A type of focus point that actually consists of two phase-detect focus points in one, which allows the focus point to detect image detail of any shape. By comparison, a non-cross-type AF point is only capable of detecting image detail that runs in a certain direction.
  • Single Focus: the AF mode that locks focus on your subject once and then stops, allowing you to wait for a candid moment or re-compose your image before clicking the shutter. Also known as One Shot Focus.
  • Continuous Focus: the AF mode that continuously tracks a subject, allowing you to maintain focus on a moving subject for as long as you hold down the AF button. Also known as Servo Focus, or AI-Servo
  • Dynamic Focus: an autofocus mode that allows the camera to move / change the active focus point, in order to track a subject if it moves around the viewfinder.
  • AF-ON: a customization that allows the photographer to activate autofocus using a different button other than the shutter release,
    and if desired, dis-allow the shutter release button from performing autofocus at all. Some cameras have a dedicated AF-ON button, on other cameras the function must be custom-assigned to another button. Also called Back-Button Focus.

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