Mirrorless Camera Focusing: How Does It Work?

Mirrorless cameras have a different autofocus system than digital SLRs. In a digital SLR, most of the light coming through the lens is reflected by the mirror into the viewfinder, and a small part is deflected downwards to a dedicated autofocus sensor. This is called phase detection autofocus. It is fast and good at tracking moving subjects, but it lacks accuracy. Mirrorless cameras, on the other hand, take a reading directly from the sensor. The camera adjusts the lens until maximum contrast is achieved, which is called contrast-detect autofocus. This system is slower, but much more accurate for still subjects.

Phase detection autofocus

Inside your camera, there are prisms that split the image you see into two images. These images are then projected onto a sensor. If these images line up, your subject is in focus. If they don't, your subject is out of focus. The camera's sensor is aware of which split image is which, so it can communicate with the camera and let it know in which direction to move the focus to ensure the images line up.

Light rays pass through the lens, and this light is detected by the autofocus (AF) sensor. The autofocus system can then determine if the subject is in front of or behind the focus point. The camera receives direct information on how it should turn the focus ring to lock onto the subject. This is how mirrorless cameras, which are smaller and lighter, achieve the same result.

Once the images are aligned, the system sends a confirmation message that the subject is in focus. All of this complexity happens in a fraction of a second, making the phase detection AF system excellent for capturing moving subjects.

Contrast detect autofocus

Contrast-based autofocus, also known as contrast detection autofocus (CDAF), is a system used in modern cameras, including mirrorless cameras, to focus on a subject. This system works by considering the portion of the image with the greatest contrast and then adjusting the focus until that particular area appears sharpest.

Imagine manually focusing on a subject by rotating the focus ring until the subject is in focus. Contrast-based autofocus works in a similar way, but instead of relying on a person's judgment, it uses the camera's sensor to evaluate the contrast in the scene. It incrementally adjusts the lens, seeking the point where contrast is highest, indicating the sharpest focus.

Contrast detection autofocus is excellent for still subjects, as it can accurately focus on the desired area. However, when shooting moving subjects, it may struggle due to its slower speed compared to other autofocus systems.

Contrast detection autofocus is highly accurate because it analyses data directly from the imaging sensor, resulting in precise focus with minimal errors. Additionally, it allows for a high number of autofocus points, enabling better tracking and precision.

In mirrorless cameras, contrast detection autofocus is utilised in conjunction with the camera's sensor. It evaluates the data captured by the sensor and incrementally adjusts the lens's focus until the desired level of contrast is achieved, indicating that the image is now in focus.

While contrast detection autofocus has been traditionally associated with mirrorless cameras, newer technologies have led to the development of on-sensor phase detection autofocus systems. These hybrid systems combine the speed of phase detection autofocus with the accuracy of contrast detection, offering the best of both worlds.

Back-button autofocus

By default, when you half-press the shutter release button, the camera's focus system tries to focus on the subject. Applying a little more pressure captures the image. This means that the shutter button is used for two different purposes: focusing and taking a picture. This can lead to accidentally taking unwanted images if you apply more pressure than intended.

With back-button autofocus, you can simplify this process. You only need to engage the focusing button when you need to focus, refocus, or continuously track a subject. The shutter release button is then used solely for capturing the image. There is no need to remember which focusing mode you are using, as you can keep your camera in AF-C mode all the time.

One of the main advantages of back-button autofocus is the ability to shoot in AI-Servo/Continuous Servo (AF-C) mode while still enjoying the benefits of One-Shot/Single Servo (AF-S). Since the dedicated focusing button is only activated when needed, you can keep your finger off it when you want to lock the focus. This is especially useful when shooting a series of portraits of a person, allowing you to focus on the subject with the rear button and then recompose the shot as desired.

Additionally, back-button autofocus provides instant manual focus control. Since the camera's autofocus is only engaged when you press the dedicated focus button, you can manually adjust the focus on your lens without the camera overriding it when the shutter is released. This can be beneficial in situations where your subject is heavily obstructed by objects that interfere with the camera's focusing system.

However, one of the drawbacks of back-button autofocus is that it requires excellent coordination to operate smoothly. It may take some time and practice to master using two buttons instead of one. Additionally, in challenging shooting conditions, such as using a heavy lens or when you are tired, it can be more difficult to maintain the coordination needed for back-button autofocus.

To set up back-button autofocus on your camera, you will need to access the Custom Functions or Custom Settings Menu, depending on the camera model and brand. For Canon DSLR and mirrorless cameras, you would typically set the shutter button to "metering start" to disable autofocus and then set the AF-ON or AE Lock button to "Metering and AF start." Similar adjustments can be made for Nikon, Sony, Fujifilm, and Panasonic Lumix cameras.

Autofocus micro-adjustment

Autofocus microadjustment (AFMA) is a feature on certain camera models that allows the user to correct small front- or back-focusing errors. It is a way of fine-tuning how your AF lenses communicate with your camera to produce the most accurate focus possible.

AFMA is important because if there is a problem with the calibration of either the lens or the camera, your images will not be as sharp as they could be. As sensor resolution increases, autofocus errors become more evident. Photographers are used to viewing images at 100%, especially if they need to print them large, and doing so will reveal focusing issues in the image.

AFMA is particularly beneficial for lenses with fast apertures (f/2.8 and wider) because the thin depth of field at wide apertures makes even small focus errors more evident.

There are two types of autofocus microadjustments: "Adjust by lens" and "All by the same amount". The former allows you to enter information for individual lenses, which can be stored in the camera. The latter makes a common adjustment in the camera, irrespective of the lens used.

To perform an AFMA, you will need a camera with the feature, a lens, a tripod, and a three-dimensional high-contrast object to photograph. The process involves focusing the camera on the object, taking test shots, and then reviewing the images to determine if any adjustments are required. If adjustments are needed, this can be done through the camera's autofocus microadjustment settings.

It is important to note that AF microadjustment is not a cure-all for focusing problems. It does not solve optical abnormalities or fix physical defects in your camera or lens. If you encounter a problem that you cannot solve, it is best to send your equipment to the manufacturer for qualified service.

Manual focus

One of the benefits of using manual focus on a mirrorless camera is the ability to adapt old school manual focus lenses from other camera systems. With the right adapter, you can mount lenses from different brands and even older SLR lenses onto your mirrorless camera. This opens up a world of creative possibilities and allows you to take advantage of vintage lenses with unique characteristics.

When using manual focus lenses on a mirrorless camera, you may encounter some challenges. Some mirrorless cameras may not provide feedback on your focus, making it more difficult to achieve precise focus. However, modern mirrorless cameras often offer features like focus peaking and viewfinder magnification to assist with manual focusing. These tools can help you fine-tune your focus, but they may also slow down your workflow.

Additionally, when adapting older manual focus lenses, you may need to manually adjust the aperture, and there may be no coupling with the aperture ring, requiring more steps in your photography process. It's important to note that the experience of using manual focus lenses can vary depending on the brand and model of your mirrorless camera. Some cameras may offer better support for manual focus lenses than others.

Overall, manual focusing on a mirrorless camera can be a rewarding experience for photographers who are willing to take the time to learn and adapt their techniques. It provides more control over your images and allows you to explore a wide range of lens options, including vintage lenses that can add unique character to your photographs.

Frequently asked questions