Mastering Auto Focus: The Magic Behind Sharp Camera Images

Autofocus (AF) is a feature in most modern cameras that helps capture sharp images. The autofocus system in a camera intelligently adjusts the camera lens to obtain focus on a particular subject. The camera's AF system relies on one or more sensors to determine the correct focus. The two main types of AF systems are passive and active. Passive AF systems use either contrast detection or phase detection methods to achieve focus. In contrast detection, the camera lens is moved back and forth until the position of maximum focus is found for the object. In phase detection, the camera analyses the light as it passes through the lens and compares light from two apertures on opposite sides of the lens. Active AF systems, on the other hand, first measure the distance to the subject and then adjust the focus accordingly.

Active vs. Passive Autofocus

There are two types of autofocus (AF) systems: Active and Passive. These systems help to focus the camera lens to obtain the sharpest image possible.

Active AF

The Active AF system works by emitting a red beam of light (or infrared light in newer cameras) onto the subject. The light is then bounced back to the camera, which calculates the distance between the camera and the subject. Once the distance is known, the camera instructs the lens to adjust its focus accordingly. Active AF is useful in low-light environments where passive AF often fails. However, it is limited to stationary subjects and only works for objects that are relatively close to the camera.

Passive AF

The Passive AF system works very differently. Instead of using a red beam to calculate the distance, it relies on Phase Detection or Contrast Detection to detect contrast. Phase Detection uses an array of microlenses to split light into a pair of images. The distance between these images is then measured to determine how far the lens needs to be adjusted. On the other hand, Contrast Detection uses software algorithms to "probe" through areas of an image for edge detail and adjust focus accordingly. Passive AF is generally slower but can be more reliable and accurate in low-light conditions.

Hybrid AF

Some cameras combine both Active and Passive AF systems to compensate for their individual weaknesses. This hybrid system can provide faster and more accurate results, especially in challenging lighting conditions.

Autofocus Sensors

A camera's autofocus sensor is responsible for achieving accurate focus. These sensors are laid out in various arrays across the image's field of view, with each sensor measuring relative focus by assessing changes in contrast at its respective point in the image. The assumption is that maximal contrast corresponds to maximal sharpness.

There are two main types of autofocus systems: active and passive. Active AF emits a signal, such as infrared light or ultrasonic sound waves, to illuminate or estimate the distance to the subject. Passive AF, on the other hand, uses contrast sensors within the camera to determine focus. While active AF works well in low-light conditions, it can be fooled by obstacles like windows or glass. Passive AF relies on sufficient light and image contrast to function properly.

Most modern SLR cameras use through-the-lens optical sensors for autofocusing, which is generally faster and more precise than manual focus. Some cameras also have autofocus assist beams, which are a form of active autofocus that helps the sensors detect the subject in low-light or low-contrast situations. However, this method tends to result in slower autofocus.

Smartphone autofocus typically uses the "phase difference detection" method, where a portion of the pixels on the image sensor is dedicated to detecting the optimal focus point. More advanced systems, such as Sony's "all-pixel auto-focus," utilise all imaging pixels for phase difference detection, resulting in stable autofocus operation even in challenging lighting conditions.

Factors Affecting Autofocus Performance

The photographic subject can have a significant impact on autofocus performance, often more so than variations between camera models, lenses, or focus settings. The three most important factors influencing autofocus are light levels, subject contrast, and camera or subject motion.

Light levels and subject contrast are interdependent factors. For instance, autofocus can be achieved for a dimly lit subject if it has high contrast, and vice versa. Selecting a focus point that corresponds to a sharp edge or pronounced texture can improve autofocus performance.

In the case of fast-moving light sources in the background, focusing on them may result in an out-of-focus subject when the depth of field is shallow. Focusing on the subject's exterior highlight or a lower-contrast, stationary point like the subject's foot or nearby objects at the same distance may be more effective.

The number and type of autofocus points available on a camera model also influence autofocus performance. High-end SLR cameras may have 45 or more autofocus points, while other cameras may have as few as one central AF point. The accuracy of autofocus points can also be affected by the maximum aperture of the lens being used.

The autofocus mode is another factor. The one-shot mode, suitable for still subjects, may result in focus errors for fast-moving subjects as it cannot anticipate their motion. On the other hand, the AI Servo or continuous mode, available on Canon and Nikon cameras, respectively, predicts the subject's future position and adjusts the focus accordingly, making it more suitable for moving subjects.

The autofocus assist beam, found on many cameras, is a form of active autofocus that uses a visible or infrared beam to help the autofocus sensors detect the subject. While this can be useful in low-light conditions or when the subject has insufficient contrast, it also slows down the autofocus process.

Number and Type of Autofocus Points

The number of autofocus points is important. The more autofocus points, the easier it is to track and maintain focus on a moving subject. This is why more expensive DSLR cameras have more autofocus points. However, it is not just the number of autofocus points that matter, but also their type.

There are three types of autofocus points: vertical, horizontal, and cross-type. Vertical autofocus points detect contrast on vertical lines, while horizontal autofocus points detect contrast on horizontal lines. Cross-type autofocus points are more accurate as they can detect contrast on both vertical and horizontal lines. They are also better at retaining focus but come at a higher cost.

When looking through the viewfinder, you may not be able to see all the autofocus points. Some of them are small and located between the larger points. The camera can only focus where there is an autofocus point, so cameras with fewer points will have larger gaps between each point, making it harder to achieve and maintain accurate focus.

For most photography, it is recommended to use only the centre autofocus point (single-point AF). This allows you to be precise about what you want to be in focus. After achieving focus, you can reframe your shot while keeping your finger half-pressed on the shutter button.

For tracking moving subjects, dynamic area AF is better. This allows you to choose between 9, 21, or 51-point placements. Using more points can slow down the autofocus system, so it is recommended to stick with 9 or 21 points.

For stationary or slow-moving subjects, single-point AF is more suitable. This allows you to ensure focus on a specific part of the subject, such as the eyes.

Autofocus Modes

Autofocus (AF) is a feature on most modern cameras that improves image quality and saves time. There are two main types of autofocus: active and passive. Active AF works by emitting a signal (e.g. a red beam or infrared light) to illuminate or estimate the distance to the subject. Passive AF, on the other hand, uses either phase detection or contrast detection to determine focus.

There are four primary autofocus modes: Continuous, Single, Automatic, and Manual. Continuous mode (also known as AI Servo AF or AF-C) is useful for keeping moving objects sharp within the viewfinder as the camera detects the subject's movements and refocuses accordingly. Single mode (also known as One-Shot AF or AF-S) focuses on the subject just once when the shutter release is depressed halfway. Automatic mode (AI Focus AF or AF-A) is a newer feature where the camera's focusing computer switches between continuous and single autofocus depending on the situation. Lastly, Manual mode requires the photographer to use the distance measurements on the lens barrel and even measure the distance from the lens to the subject to achieve perfect focus.

Different camera brands have different names for their autofocus modes. For example, Canon uses "One-Shot AF" for single autofocus mode, while Nikon uses "AF-S". It is important to refer to your camera's manual to understand the specific autofocus modes available on your device.

Frequently asked questions