The Evolution Of Autofocus In Mirrorless Cameras

Mirrorless cameras have autofocus capabilities, but they work differently from DSLRs. DSLRs use a phase detection autofocus system, which is generally faster and better at tracking moving subjects. However, mirrorless cameras use a combination of phase detection and contrast detection autofocus, which is much more accurate, especially for still subjects.

Mirrorless cameras, such as the Sony A7, do not use a traditional phase detection autofocus system. Instead, they rely on information from the camera's sensor to enable features like eye-tracking autofocus. While this can be advantageous in certain situations, it also comes with trade-offs. For example, mirrorless cameras may struggle in low-light conditions due to the infrared filter, which DSLRs can work around using an AF assist light.

The autofocus system in mirrorless cameras is a hybrid of phase detection and contrast detection. In phase detection, the camera's pixels are split in half to distinguish light from different directions, allowing the camera to focus accurately. Contrast detection, on the other hand, involves moving the lens back and forth to find the sharpest point.

Overall, while mirrorless cameras may have some autofocus limitations, they excel in certain areas, such as accuracy for still subjects and eye-tracking autofocus.

Phase detection vs contrast autofocus

The autofocus system your camera uses makes a big difference to its effectiveness. There are two main types of autofocus (AF): contrast detection and phase detection autofocus. Both have their pros and cons, and the one you choose depends on the type of photography you want to shoot.

Contrast Detection AF

Contrast detection autofocus uses the contrast between edges in your scene to find focus. It analyses the contrast on pixels on the camera's sensor and pushes the lens back and forth until it finds the right focusing point. When contrast is at its highest, the subject is in focus.

Contrast detection is exceptionally accurate since it analyses every point of contrast, pixel by pixel, on the camera's sensor. However, this process is time-consuming, making it a slow way of autofocusing.

Contrast detection is smaller, less expensive, and more accurate than phase detection AF, especially with still subjects. But it's also slower. It takes time for a contrast detection system to calibrate, so it's not ideal for fast-moving subjects.

Phase Detection AF

Phase detection autofocus calculates the camera-to-subject distance and tells the lens where to focus. It does this by separating the light entering the camera into two beams. When the two parts converge, the image is in focus. When they don't line up, the system makes precise changes to the focus motor to align the images.

Phase detection is much faster than contrast detection, able to autofocus in a fraction of a second. However, it is also more costly and heavy.

Phase detection is very good at tracking moving subjects, which is why it is considered the best system for SLRs. However, it is less accurate than contrast detection. It doesn't work as well in low light, and may struggle with low-contrast subjects.

Hybrid Autofocus Systems

Some cameras use a hybrid of the two systems. For example, a digital SLR camera may use contrast detection AF in Live View or movie mode and phase detection AF when you look through the viewfinder.

Some mirrorless cameras also have phase detection AF points that work in continuous AF mode to improve AF tracking accuracy.

Mirrorless Cameras and Autofocus

Mirrorless cameras don't have a dedicated autofocus sensor but take readings from the main sensor. This means they use contrast detection AF. However, most mirrorless cameras will also use phase detection points built into the sensor, with contrast AF to supplement it.

Mirrorless cameras are considered better than SLRs when it comes to focusing on still subjects. They are also better for using manual focus lenses.

Tracking moving subjects

Mirrorless cameras use the imaging sensor to gather information for autofocus, whereas DSLRs use a dedicated autofocus sensor. This difference has several implications for tracking moving subjects. Firstly, mirrorless cameras typically have autofocus points that go closer to the edge of the frame, making it easier to focus on off-centre subjects without having to focus and recompose. Secondly, mirrorless cameras have access to all the information coming off the sensor, enabling features like eye autofocus (eye AF), which allows for precise focus on a subject's eye across the frame. This is especially useful for portrait, pet, wildlife, sport, wedding, and social photography.

That being said, DSLRs still have advantages when it comes to tracking moving subjects in certain situations. Phase detection autofocus, which is used in DSLRs, is generally faster and better at tracking moving subjects than the contrast detection autofocus used in early mirrorless cameras. However, most modern mirrorless cameras combine contrast detection with phase detection to create a hybrid autofocus system, improving their performance in this area.

When it comes to specific camera models, several options stand out for their autofocus capabilities when tracking moving subjects. The Sony A1, Canon EOS R3, and Nikon Z9 are currently considered the best professional-grade mirrorless cameras in this regard. The Sony A1, in particular, has been praised for its Real-time Tracking AF, which utilizes artificial intelligence to seamlessly track moving subjects. Other notable options include the Canon EOS R5, Sony A7R IV, and Nikon Z7 II.

Overall, while DSLRs may still have the edge in certain situations, mirrorless cameras have made significant strides in autofocus performance for tracking moving subjects. The latest mirrorless models offer a range of advanced features and improvements that make them a compelling choice for photographers who need to track moving subjects.

Autofocus for still subjects

Autofocus is a handy feature for photographers, and it is available on mirrorless cameras. However, it is important to know how to use autofocus effectively, especially for still subjects.

The autofocus mode you should use depends on the type of subject you are photographing. For still subjects, the "`one-shot`" autofocus mode is the best option. This mode ensures that the camera achieves a focus lock before taking the photograph. It is important to note that the "one-shot" mode is not suitable for fast-moving subjects as it cannot anticipate their motion.

When using autofocus for still subjects, it is recommended to use a single, well-placed autofocus point. You can compose the frame first and then place the autofocus point where you want the camera to focus. This gives you more control over the focal point of the image.

Additionally, it is important to consider the lighting conditions and the contrast of the subject when using autofocus. Autofocus works by assessing changes in contrast, so having a subject with good contrast and lighting will help the camera achieve accurate focus.

Another factor to consider is the autofocus sensor. Some cameras have multiple autofocus sensors that work together to improve reliability, while others have a single autofocus sensor. The number and type of autofocus sensors can impact the accuracy of the autofocus.

Overall, by understanding the limitations and capabilities of autofocus, photographers can effectively utilise this feature to capture sharp images of still subjects.

Face and eye detection

When using Face and Eye Detection AF, you'll see detected faces in the viewfinder even before you’ve focused on them, highlighted with boxes. A green box highlights the face currently being tracked as a priority. If there are any other faces in the frame, these may be highlighted with white boxes. Some cameras let you switch between which face is being tracked just by rotating the rear command dial, which is easy to do with the camera at your eye.

When the subject is large enough in the frame, you’ll see a yellow box visible over the closest eye to the camera. Depending on the distance to your subject, you may see the yellow box over the face, instead of an eye. When more than one eye is visible to the camera, you’ll see a small triangle to the left or right of the eye, depending upon where the eye is in relation to the subject’s other eye. When you see the triangle, you can use the multi-selector (on the rear of the camera body) to move the Eye-Detection Autofocus to the other eye using the left and right buttons of the multi-selector.

Autofocus issues with mirrorless cameras

Mirrorless cameras have autofocus issues, especially when compared to DSLRs. However, it's important to note that DSLRs also have their own set of drawbacks when it comes to autofocus.

Phase detection autofocus

Phase detection autofocus is the system used by DSLRs. It works by directing a small portion of the incoming light through a mirror and into a dedicated autofocus sensor. This system is very fast and excellent at tracking moving subjects. However, it lacks accuracy due to a combination of factors, including the use of non-cross-type autofocus points and the need for precise calibration of the camera and lens.

Contrast detect autofocus

Mirrorless cameras, on the other hand, use a contrast detect autofocus system. This system takes a reading directly from the sensor and adjusts the lens until maximum contrast is achieved at the desired focus point. This method is much more accurate, especially for still subjects, but it is slower than phase detection.

On-sensor phase detection

Some mirrorless cameras use on-sensor phase detection, which offers near-DSLR levels of performance, especially in bright light. This system also provides higher accuracy for still subjects and face/eye recognition. However, it cannot use IR AF assist, and in low-light conditions, the system may fail to find focus, falling back to the slower contrast AF method.

Depth-from-defocus

Depth-from-defocus is a variation of contrast AF developed by Panasonic. It uses known information about lens characteristics to supplement contrast AF techniques.

Trade-offs

While mirrorless cameras offer advantages such as better accuracy and a higher number of autofocus points, they are currently at a disadvantage when it comes to tracking moving subjects in low-light conditions. This is because they cannot utilise IR AF assist lights in the same way as DSLRs.

Frequently asked questions