At its most basic, an autofocus system is designed to ensure that the subject of your photo is sharp and not blurry by predicting what you're trying to focus on. It's a feature that most cameras are equipped with, apart from some purpose-built cameras like the GoPro HERO8. Autofocus systems vary from camera to camera, and some are better suited for different kinds of uses, but at their core, they all work with the same principle: sensors inside the camera measure how far away a subject is from the camera, then adjust the lens' focusing ring to take a clear image.
If you're looking to quickly capture a clear image without manually adjusting focus, which can be a necessity when taking pictures of a moving object, an effective, reliable system is crucial. An improperly-focused image is almost impossible to fix in the editing stage, so if you're looking to take better pictures more consistently, it's important to understand the uses and limitations of your camera's autofocus system.
While most cameras currently on the market have some form of autofocus feature, their design can vary drastically from camera to camera. There are essentially two types of autofocus system: phase and contrast detection. In the former, which is what DSLR cameras use when looking through the viewfinder, a small part of the light entering the camera lens bypasses the main focusing mirror and is bounced off of a secondary mirror. It is then divided and directed toward multiple sensors in the camera, each of which constitute a single focus point. These divided portions of light each create their own distinct images, which the camera then compares. It then proceeds to adjust the lens' focusing ring until all of these distinct images look identical. Once that occurs, the image is considered to be 'in focus'.
Contrast detection systems work differently. This design works by having the camera constantly monitor the degree of contrast in an image, as peak contrast levels result in a clear distinction between different objects in your photo that equates to a sharper image. Since this system doesn't use individual sensors, cameras using this technology can have an extremely high number of focus points without the associated hardware, resulting in a smaller physical package. This gives you a larger degree of freedom in being able to change what is rendered sharply in your image without forcing you to move in order to recompose your shot, or maintain focus on a subject as they move across the frame. However, this system can be slower than phase detection, since the system is constantly comparing focus distances in order to determine peak contrast, though this difference in speed may only be noticeable on older systems. Mirrorless cameras tend to use a combination of both systems, which is referred to as hybrid autofocus. These systems use phase detection using information acquired directly from their sensor to quickly acquire an approximate focus point and then use the more sensitive, but slower, contrast detection system to bring the image into sharper focus.
Our tests are used to measure both an autofocus system's given features as well as a rough approximation of its overall performance. It's worth noting that autofocus performance can vary drastically depending on a multitude of factors. Lighting conditions can have a big impact on overall performance since most autofocus systems rely on having enough light to determine what is and what isn't in focus.
Your choice of lens plays a large part as well. Its maximum aperture influences how much light enters the camera, which governs autofocus behavior. While a wider aperture lets in more light, it also creates a shallower depth of field, increasing the chances of an image being out of focus. Conversely, using a narrow maximum aperture lets in less light, which could negatively impact overall performance, but also results in a deeper depth of field, reducing the chances of an image being out of focus. The type of focusing motor the lens uses is also important, as some operate significantly faster and more smoothly than others.
The advertised coverage field shows how much of the frame is covered by focus points. Having a large coverage area allows the camera to maintain focus on a subject, no matter where they are in the shot. This figure can be obtained from the camera's list of specifications.
The advertised focus points test simply shows how many different focus points a camera's autofocus system has. This number is obtained from a camera's list of specifications.
Since traditional SLR cameras are limited to the number of autofocus sensors that are physically located inside their body, the number of focus points tends to be much lower than that of mirrorless cameras. For instance, the Canon Rebel SL3, a traditional DSLR, has only 9 autofocus points while a comparable mirrorless alternative, the Canon EOS M50, features 143, giving you a greater amount of creative flexibility in choosing what the focal point of your image is. This advantage in flexibility is skewed even further in favor of mirrorless cameras when considering more expensive models, like the Canon EOS R, which has an autofocus system with 5655 detection points.
The manual focus assistance test indicates whether or not the camera is fitted with an aid to help you ensure that your shot is sharp when you're adjusting the focus by yourself. This aid can come in many different forms, such as focus peaking, which brightly highlights parts of the image that are in focus, and focus magnification, which zooms in on part of the image and makes it easier to see if it's sharp. The presence of this feature can be determined by looking at a camera's specifications.
This feature is commonly found on most cameras, even basic point-and-shoots like the Canon PowerShot SX740.
The face detection distance test is used to evaluate the maximum distance at which the camera is capable of detecting a face. To perform the test, the camera is held still on a tripod and its lens' focal length is set to the full-frame equivalent of 30mm. The camera is set to AF-C (continuous autofocus) and tracking mode with face and eye detection (if applicable) is enabled. The tester then walks up to a photo portrait on a wall to see if eye detection works before beginning to move backward, whereupon the detection system should switch from eye to face detection.
Once the camera no longer detects a face, the tester points it away from the portrait before orienting it back, though the tester doesn't do this more than twice. If the camera doesn't detect a face, the tester will move forward until it's capable of doing so. When the camera locks on to the subject consistently, the distance is noted.
The eye detection field shows whether or not an autofocus system has an eye-detection feature. This feature is used to identify a subject's eyes and maintain focus on them and is commonly found on most cameras currently on the market. However, some basic point-and-shoots, like the Canon PowerShot SX740, don't have it. In non-mirrorless models, the camera can be used in LiveView mode to enable this feature. The presence of this feature can be determined by looking at a camera's specifications.
The face tracking success rate test is used to monitor the ratio of clear to blurry photos while using the camera to take photos of a moving subject that's approaching the camera head-on. To perform this test, the camera is positioned on a tripod with the screen at eye level. The autofocus system is set to AF-C (continuous autofocus) mode. Photos are taken while the camera is in full manual mode, with a shutter speed of 1/100s to reduce the chances of blurry images being caused by motion blur. Focal length and aperture can vary depending on sensor size, with full-frame cameras using respective values of 50mm and F5.0 and APS-C cameras being set to 33-34mm and F3.5. This difference is meant to compensate for the fact that smaller camera sensors generate a deeper field of view when using the same aperture and focal length. Camera ISO can vary depending on lighting conditions but doesn't exceed ISO 800 to ISO 1200 to make analyzing the image afterward easier.
To test performance, one tester approaches the tripod-mounted camera at a steady pace while the other stands behind it and takes pictures of the approaching subject. The camera is set to a medium-speed continuous shooting mode of around 7 fps and face and eye-tracking are enabled. This process is repeated six times to create a sufficient sample size for a representative average result during analysis.
The images are analyzed to determine whether or not essential components of the image are sharp or blurry by using the Focus Mask tool in RawTherapee and the testers' subjective opinion, as the Focus Mask tool doesn't always accurately gauge what is and what isn't in focus. The score is tallied as follows. If the tester determines the subject's face to be clearly in focus, one point is given. If the face is blurred, no points are given. If the tester isn't completely sure whether the face is blurred or sharp, half a point is given.
The object tracking success rate test is used to monitor the ratio of clear to blurry photos while using the camera to take photos of an object that's approaching the camera head-on on a predetermined path. The autofocus system is in AF-C (continuous autofocus) mode, the picture format is set to 3:2, and JPEG quality is set to 'Fine Quality'. Photos are taken while the camera is in full manual mode, with a shutter speed of 1/100s. Focal length and aperture can vary depending on sensor size, with full-frame cameras using respective values of 50mm and F5.0 and APS-C cameras being set to 33-34mm and F3.5. This difference is meant to compensate for the fact that smaller camera sensors generate a deeper field of view when using the same aperture and focal length, allowing us to provide a reasonably accurate comparison between cameras with different sensor sizes.
To test performance, a coffee mug is placed on a sliding rack at the minimum distance to which the camera can focus on it. The mug is pulled gradually backward while the camera captures images in its continuous shooting mode of around 7 fps. This sequence is repeated three times to create a sufficient sample size for a representative average result during analysis. The images are analyzed to determine whether or not essential components of the image are sharp or blurry by using the Focus Mask tool in RawTherapee and the testers' subjective opinion, as the Focus Mask tool doesn't always accurately gauge what is and what isn't in focus. The score is tallied as follows. If the tester determines the logo on the coffee mug is clearly in focus, one point is given. If the logo is blurred, no points are given. If the tester isn't completely sure whether the logo is blurred or sharp, half a point is given.
As stated previously, the results of this test don't depend solely on the equipment that you're using, but also a myriad of environmental factors that can't always be accounted for, even in a relatively controlled testing environment.
As mentioned previously, the type of autofocus system your camera uses isn't the only governing factor in overall performance. Lighting conditions and the type of lens you're using are also very important. If you're looking to get the best out of your camera's autofocus system, it's best to be in a well-lit environment and for you to be using a fairly modern lens that's in good working order.
While a camera's autofocus system won't help you take better pictures, it will increase your likelihood of capturing a clear image in difficult shooting conditions, especially if your subject is moving.