CAMERA FEATURES

Everything you wanted to know about autofocus (AF)

Find out about Canon's autofocus (AF) systems, how they work and the AF options available.
Canon autofocus systems are so easy to use and so effective that many photographers rarely switch to Manual Focus or MF (not to be confused with Manual exposure mode, denoted by M in the camera menu and on the mode dial, if your camera has one). But how do they work, and what do the different options mean?

When you use autofocus, there are a range of settings and options available, which may vary from camera to camera. For easier menu navigation and setting, all the AF settings and Custom Functions are grouped into one menu tab, so there is no need to jump into different menu areas to make changes. These are the choices on the EOS R5:

AF operation: One Shot AF (for still subjects) or AI Servo AF (for moving subjects). Some cameras have an AI Focus AF mode in which the camera chooses which of these two to use, according to the subject movement it detects. Find out more about AF operation modes.

AF method:
  • Face Tracking – including, in the latest cameras, bird and animal tracking;
  • 1-point AF – the camera focuses using a single AF point;
  • Spot AF – the camera focuses using an even smaller area than 1-point AF;
  • Expand AF area – there are two options here. With either, the camera focuses using a single AF point, but if it is unsure then it uses another AF point to assist, or may switch to that point instead – either the next point horizontally and vertically, or the next point diagonally as well. Both of these are effective with moving subjects, which are difficult to track with 1-point AF;
  • Zone AF, Large Zone Vertical, or Large Zone Horizontal – uses auto selection AF within a larger area, optionally focusing on the nearest subject or using various criteria such as faces, subject motion and subject distance.
Find out more about AF methods.

Subject to detect – on the latest cameras with Intelligent AF, this instructs the camera's AI to give priority to People, Animals or No Priority. Find out more about AF configuration options.

Eye Detection on or off.

Touch & drag AF settings (available on selected newer cameras including the EOS M50 Mark II and EOS R System models) – determine whether you can move the AF point by dragging on the camera's screen (Relative) or set the AF point by tapping (Absolute).
A user's finger points to the AF/MF switch on the barrel of a Canon lens.

Switch autofocus on by moving the switch on your lens, if there is one, from MF (manual focus) to AF. Otherwise, depending on the camera, choose AF under Focus Mode in the camera menu, or else use the cross-keys or the dedicated switch on the camera, if there is one.

A finger presses the AF-ON button on the back of a Canon EOS-1D X Mark III and the image on the rear screen shows the camera focusing on a bird in flight.

To set the desired AF setting, switch to AF, select the desired shooting mode (AF will operate only in the automatic and semi-automatic modes) and press the AF button on the camera. Hold the AF-ON button until the camera achieves focus – when autofocus is achieved, the AF point will turn green if you're using One-Shot AF mode or blue in Servo AF mode.

How AF works in a DSLR or a mirrorless camera

If you use a modern EOS DSLR, you may be unaware that it actually has two different AF systems. One is used when images are composed in the viewfinder, and the other is used in Live View or video mode when the image is composed on the screen on the back of the camera. Mirrorless cameras, including the full-frame mirrorless EOS R System cameras, work in the equivalent of Live View mode all the time, so they have just one AF system.

When you're using the viewfinder in a DSLR, the main reflex mirror reflects light into the viewfinder. A sub-mirror behind the main mirror reflects some light into a dedicated autofocus sensor. In Live View or video mode, the main mirror lifts up out of the optical path so that the imaging sensor receives light all the time, not just during the exposure. In this mode, the DSLR uses the imaging sensor to gather autofocusing data. Mirrorless cameras use only this system.

When the viewfinder is in use, EOS DSLRs use highly sensitive line sensors for autofocus ranging. Called BASIS (BAse Stored Image Sensor), these consist of two 48-bit line sensors and associated amplifier circuitry.

The sensors are in the base of the camera. As the sub-mirror behind the camera's reflex mirror reflects light down to these sensors, this light is split by a small lens assembly to form two separate images. One image is formed on the first line sensor, the other on the second line sensor. If there is no deviation between the two images seen by the sensors, the camera knows the lens is focused. However, if the spacing of the two images is not correct, a signal is sent to the lens motor to bring the subject into sharp focus.

DSLRs in Live View mode and mirrorless cameras apply a similar principle but using data from two points on the imaging sensor.

Dual Pixel CMOS AF

In Live View and video mode, the most affordable EOS DSLRs use a contrast-detection autofocus system. This works on the principle that the image is sharpest when there is greatest contrast between adjacent pixels. This system will typically adjust the focus rapidly back and forth a few times to determine the point of peak contrast. However, EOS mirrorless cameras and most EOS DSLRs use a different principle, phase-detection, drawing on a special feature of their imaging sensor's design: Canon's Dual Pixel CMOS AF system, introduced in the EOS 70D in 2013.

Each pixel on the Dual Pixel CMOS sensor has two independent photodiodes (the parts of the sensor that record light intensity or brightness). The camera's processor compares the signals from the two photodiodes, and if they match, it knows that this area of the image is in focus. If there is any deviation between them, it looks at pairs of photodiodes across a group of pixels, and can then calculate which direction the lens needs to be adjusted to achieve sharp focus, and how much focus adjustment is required. In this way, Dual Pixel CMOS AF phase-detection focusing usually requires less trial-and-error and is more effective than contrast detection.

What's more, where other AF systems use only a limited number of dedicated individual pixels for phase-detect AF, Dual Pixel CMOS AF uses every pixel on the imaging sensor, which means that the active AF area covers in effect the entire image frame. It also gives the camera a significant advantage for tracking a subject around the frame, because there are no gaps between the AF points. It offers huge advantages for video, including smooth tracking of moving subjects and dazzling pull focus effects with touch screen control, and the technology is used in Canon's Cinema EOS professional cine cameras.
A diagram representing light hitting the two photodiodes in a pixel in Canon's Dual Pixel CMOS AF system.

In Canon's unique Dual Pixel CMOS AF system, every pixel in the sensor is capable of both imaging and phase-detection autofocus. Each pixel has two independent photodiodes, labelled A and B here, and if there is any deviation between the two signals, the camera knows that this point of the image is not in sharp focus. By looking at pairs of photodiodes across a group of pixels, it can determine how much adjustment is required to achieve sharp focus and in which direction.

A cutaway diagram of the light path in a DSLR, with light entering the lens and being reflected up to the viewfinder and also down to the camera's autofocus module.

When you use a DSLR viewfinder, the dedicated autofocus sensor at the bottom of the camera receives light reflected down from a secondary mirror. When you switch to Live View, the imaging sensor is used for both imaging and focusing.

Autofocus modes

Most EOS cameras offer two different autofocusing modes, and some offer three. Although the end result is that the lens automatically focuses, you'll get best results by setting the mode to suit the subject.

One-shot AF

One-shot AF mode suits most subjects that stay in one place while you take a photograph. The focus is locked with the first pressure on the shutter button.

One-shot AF is best if you don't know which mode to use – it's a good general-purpose setting to suit most subjects. In practice, you compose your subject in the viewfinder and half-press the shutter button. Among other things, this activates the autofocusing. The lens will focus on the subject, and then lock. A green focus confirmation signal will appear in the viewfinder to tell you focus has been achieved, and the in-focus beeper will sound (unless you have deactivated it).

As long as you keep partial pressure on the shutter button, the focus will not change, even if you move the camera to recompose the shot. This gives a very quick and convenient method of achieving focus lock. If focus is not achieved, the AF point will turn orange.

In One-Shot AF mode, the camera will not let you fully depress the shutter button to fire it unless the subject is in focus. This means that if the camera is unable to focus the lens, you will not be able to take a picture.

AI Servo AF

AI Servo AF mode is designed for fast-moving subjects. The camera calculates where the subject will be at the moment the shutter fires and focuses the lens accordingly.

AI Servo AF does not utilise focus lock as One-Shot AF does. It continually checks the focus and refocuses the lens each time the camera-to-subject distance changes, right up to the moment of exposure. This makes it ideal for photographing moving subjects − you can retain partial pressure on the shutter button as you follow the subject with the camera, applying full pressure to take a picture at the key moment.

When focus is achieved in AI Servo AF mode, the AF point will turn blue. But one potential problem is that AI Servo AF allows the shutter to be fired even if the subject is not in focus. If the lens has not finished refocusing or has failed to find the focus, you will end up with an unsharp image.

Since the system is a predictive one, it continually calculates the next position of the subject being tracked by comparing focus distance results as they are received. The algorithm ignores a reading if it is significantly different from what is expected based on other results. This helps to reduce the lens jumping completely out of focus.

AI Focus AF

AI Focus AF mode switches between One-shot AF and AI Servo AF according to the movement of the subject. The camera makes the decision.

One-shot AF is good for static and slow-moving subjects; AI Servo AF is better for subjects moving at speed. But when should you switch? The camera works that out. If AI Focus AF is selected, the camera will automatically switch from One-shot AF to AI Servo AF mode when it detects subject movement of a certain speed.

The camera detects movement by taking several AF readings as the shutter button is partially pressed. If the subject distance changes between readings, the system concludes that the subject must be moving. The variation between distances allows the camera to determine the speed of movement.

If you mostly shoot landscapes and other static subjects, AI Focus AF could be a good default setting for your camera. The odd times when you encounter a subject travelling at speed, you won't have to remember to change the AF mode. Most photographers shooting sports and wildlife prefer to set AI Servo AF.
A Canon EOS RP on a tripod, with the AF point on the rear screen picking out a bee on a flower.

Using Single Point AF or 1-point AF enables you to target the part of the subject that you want the camera to focus on. Single Point Spot AF or Spot AF sets the camera to use an even smaller area of the AF sensor, making it ideal for targeting a very small point of interest such as the insect being photographed here.

A windsurfer on the rear screen of a camera with three AF points active out of nine.

In AI Servo AF mode, the camera will continuously adjust the focus to follow the movement of the subject. Even in this mode, though, it's important to keep the AF point(s) positioned over the subject. You can leave all the AF points active and allow your camera to select the appropriate one(s), or you can manually select a single AF point. The latter is best when the subject is small in the frame or when the background would make it challenging for the camera to pick out the subject with all the AF points active. Your camera may allow you to set smaller groups or zones of AF points. These are useful when the action is unpredictable and where it would be too difficult to keep a single AF point over the subject as it moves.

Predictive focusing

If you're photographing moving subjects, having the lens focus on the subject as you press the shutter button is not ideal. It does not take account of "shutter lag" – the very brief amount of time between pressing the button and the shutter actually opening. During this time, the reflex mirror inside a DSLR has to swing up to allow light passing through the lens to reach the sensor at the back of the camera. Mirrorless cameras also suffer from shutter lag – when using mechanical shutter, the shutter mechanism has to close and then open again for the exposure.

Shutter lag on modern cameras is very short − typically around 55 milliseconds for one of the professional cameras, up to about 144ms for one of the entry-level models. But let's take an average of 100ms and see how far a subject can move in this amount of time. Someone walking at a speed of 5km/h covers 1.4m a second. In a tenth of a second (100ms), they will cover 0.14m or 14cm. This is unlikely to have a major impact on the focus. But now imagine you are photographing a racing car travelling at 200km/h. This is 40 times the speed of the walker, so the distance covered in a tenth of a second will be more than 5m. This could easily throw the image seriously out of focus.

In its cameras, Canon overcomes this problem with predictive focusing. After making several readings in AI Servo AF mode, the camera is able to determine the speed and direction of travel of a moving subject. It can then build this information into the instructions passed to the lens, so that the lens focuses on the point where the subject will be as the shutter opens.

When AI Servo AF is set, the camera continuously records the position of the subject and predicts where it will be for the next frame, based on its motion so far. If the camera fails to detect the subject position in one recording period, the AI Servo AF algorithm will ignore the negative result and the next focus point is based on the previous accurate results. It will also ignore the results when the AF distance appears to jump greatly, so that it can continue to track a subject even if an obstacle passes between you and your subject (more on this later).

Equally, if there is a sudden large jump in the focus distance, the camera will not drive the lens to the new distance directly. Instead it will gradually drive the lens focus, based on the previous successful focus distance

DSLR Focus sensors

We've noted that mirrorless cameras use the imaging sensor for focusing, but DSLRs have a separate focusing sensor. The first EOS cameras used a single AF sensor or focus point. This sensor was positioned to focus the lens on a subject in the centre of the viewfinder image. However, there are many circumstances where the main subject is not in the centre of the frame, so since the EOS 10 in 1990, EOS cameras have multiple focusing points. A number of AF sensors are positioned across the image area, each taking a reading from a different part of the scene. The camera can analyse these readings and decide which focusing point to activate. Alternatively, the photographer can select the point to use. But either way, the lens is focused using the information from the active point.

These days even an entry-level DSLR has an autofocus sensor with several AF points. The
EOS 4000D, for example, has 9 points, and the flagship EOS-1D X Mark III has 191 points. Some of these points are made up of two line sensors that create a cross, which is sensitive to both horizontal and vertical detail. Single line sensors have difficulty focusing, for example, on an unbroken pattern of vertical lines, because focusing requires them to detect changes in contrast in the vertical line they scan. Cross type AF points improve AF speed, accuracy and effectiveness in low light.

Some points, often the central point, can function as dual-cross type AF points. This means they have two AF line sensors that form an X as well as a conventional + formed from horizontal and vertical line sensors. This increases the sensitivity and precision of focus.

Some of the points behave differently depending upon the maximum aperture of the lens that's mounted (see the diagram below). As a rule, lenses with larger maximum apertures get the best from the camera's AF system.
Several stalks of purple flowers in a field, with two active AF points in green within the AF zone.

When the camera is set to select the AF point from a large area, it targets objects closest to it and near the centre of the frame.

A diagram of a sensor with 45 AF points, in different colours according to which lenses they operate wtih.

A diagram of a sensor with 45 AF points. Black: Vertical line sensors operational with lenses having an f/5.6 or greater aperture. Blue: Cross-type sensors operational with lenses having an f/2.8 or greater aperture; vertical line sensors operational with lenses having an f/5.6 or greater aperture. Red: Cross-type sensor operational with lenses having an f/4 or greater aperture; vertical line sensor operational with lenses having an effective aperture of f/8 or greater. Notice that of the seven cross type sensors, six of them (the blue ones) are functional with only a small number of lenses, mainly the fast prime lenses and the f/2.8 zoom lenses. If you are not using one of those lenses, then those cross-type sensors perform no function in autofocusing your lens.

AF point selection methods (AF method)

Although having many AF points allows the subject to be targeted precisely, there are times when it's convenient to group the points to cover a wider area, making the subject easier to locate. For this reason, EOS cameras have a number of AF point selection methods that determine how the active AF point is selected. These methods vary depending upon whether you're shooting with a DSLR in viewfinder mode or in Live View mode, or if you're using a mirrorless EOS camera, but they function in a similar way.

Using Single Point AF or 1-point AF method, the photographer can select a single AF point from all of those available for the camera to use for focusing. Conversely, in Automatic Selection, the camera selects from any of the AF points available to focus the subject.

Single Point Spot AF or Spot AF is the same as Single Point AF and 1-Point AF, but in Spot focusing method the camera uses a smaller section of the AF sensor to allow you to more precisely place the AF point on the selected subject. This is useful when shooting past obstacles, such as when focusing on an animal lying in long grass. However, Spot AF is not recommended for fast moving subjects or in very low light conditions. When you're using either of these two options the non cross-type AF points will blink during AF point selection so that you are aware if the AF point you wish to use is a cross-type point or not.

Some cameras also feature a couple of AF Point Expansion settings for more control over tracking moving subjects. In AF Point Expansion mode, a single AF point is selected manually and the camera then uses that point plus four or eight surrounding points to help track the subject. These are very useful for sports photography when you're able to keep the active area over the subject. It's easier to keep a group of AF points over a moving subject than a single AF point.

Several EOS cameras also have a Zone AF method and in some cases more than one with additional options such as Large Zone AF: Vertical and Large Zone AF: Horizontal. These options allow you to target specific areas or zones of the image frame for focusing. The photographer selects the zone while the camera selects the particular AF points to use within that zone.

The Zone AF options are useful when you know approximately where the subject will be in the frame and it would be hard to keep a smaller active area over the subject.

You can choose between AF methods by selecting AF method on the first tab of the camera's AF menu. To set the AF method, you can also press the AF point selection button and then the M-Fn button, if your camera has this. Each press changes to the next AF method.

Specifying the AF point

Although Automatic AF point selection gives good results much of the time, there are situations where it will struggle. If you frame a landscape with a nearby tree branch, for example, the camera might focus on the branch rather than the more distant scene.

For the most precise focusing, you can switch to Single Point Spot AF, Single Point AF or 1-point AF mode and select an AF point that sits over the area you want to focus on. On recent cameras including the EOS R, EOS R5, EOS R6 and EOS-1D X Mark III, you can use the initial Servo AF point to select the subject for the automatic system to track.

Alternatively, select a convenient point and use the focus-and-recompose technique described under Focus Lock below.

EOS intelligent Tracking and Recognition AF (EOS iTR AF)

EOS intelligent Tracking and Recognition AF is also available for selection on some EOS DSLRs in Automatic Selection AF and Large Zone or Zone AF mode. When this is activated, the camera makes use of data provided by the RGB AE sensor and DIGIC image processor to improve focus tracking in AI Servo mode. This means that it can use colour and exposure data to help inform the AF system.

Except when they use Dual Pixel CMOS AF, AF systems look for contrast and will focus at the area of greatest contrast. However, in some situations, especially with Auto AF point selection, this can lead to the focus jumping from the subject to a different area as the contrast levels change due to changes in lighting. Including information about the colour of a subject in the AF system's calculations improves its tracking capability. By using the colour of the subject that was initially focused on, the AF system can track the movement of that subject, both by contrast and by colour across the frame, and automatically select the most appropriate focus point given the position of the subject within the frame.

The system works not only with the colour of subjects but also with faces. Because the AE system can detect the presence of a face within the frame, the subject can be tracked across the frame accurately and quickly without having to change the focus point continually. If there are multiple faces within the frame, then you can manually select an AF point to ensure that the correct face is focused on initially and then tracked in subsequent frames.

Face Detect + Tracking and Eye Detect AF

Further developments in face recognition capability have enabled the introduction of Face Detect + Tracking and Eye Detect AF on mirrorless EOS cameras and recent DSLRs in Live View mode. In this mode the camera uses artificial intelligence to help it find faces in the scene – and if eye detection is enabled, it can find eyes in the scene and focus on the one that's selected.

The latest evolution of the system enables the camera to be set to detect people or animals (in particular members of the dog and cat families, or birds) in the frame, making it ideal for wildlife photography as well as portraits of humans or pets.

An AF point appears over any face detected, which is then tracked. If no face is detected, then the entire AF area is used for auto selection AF.
The rear screen of a Canon EOS M6 Mark II showing the camera autofocusing on the face and eye of a man wearing a blue baseball cap.

Cameras with Face Detect + Tracking and Eye Detect AF (such as the EOS M6 Mark II being used here) make it easy to capture sharp portraits – the camera will look for faces or even eyes in the frame and automatically focus on them. On some cameras, the AF point will turn blue when it has successfully acquired an eye.

A crane swoops to pluck a fish from the water.

In addition to face- and eye-detection, the EOS R5 and EOS R6 have expanded the capabilities of the AF system with animal and bird tracking, made possible by deep-learning artificial intelligence. © Robert Marc Lehmann

AF Configuration Tool

Within the AF settings of more advanced EOS cameras, there are some configuration options that enable the photographer to customise how the AF system responds to certain situations.

For example, the Tracking sensitivity control is useful for adjusting how the camera responds to objects coming between it and the subject. The standard setting is zero, and it's useful for photographing a wide range of moving subjects. Selecting the -1 or -2 (Locked on) setting tells the camera to continue to track the subject even if an object comes between it and the target. This is useful when panning, for example, when objects such as trees, lamp posts or the pillars of a stadium are likely to get in the way briefly. It can also help when photographing swimming when the subject momentarily disappears beneath the water.

Adjusting the Tracking sensitivity in the other direction, to +1 or +2 (Responsive), sets the camera to respond quickly to changes in the subject distance. This is useful when the subject is coming towards the camera quickly, or if you want the camera to always focus on the closest subject. However, if you're photographing a team sport, this can result in the focus frequently jumping between players.

The Acceleration/Deceleration tracking control determines how the AF system responds to changes in speed. With three setting levels, you can adjust the focus response for greater stability in the AF system. The 0 setting is designed for subjects that don't change their speed much during motion – for example if you're photographing racing cars or cyclists in a straight, flat part of a track. Settings 1 and 2 are designed for subjects that move suddenly or accelerate or stop suddenly. They are useful for fast-moving, unpredictable subjects such as basketball players. These settings should not be used with smooth-moving subjects as it could make the focus more unstable for those subjects.

AF point auto-switching is used in combination with Auto AF point selection, Zone AF or AF Point Expansion. It allows you to adjust the speed at which the AF points are changed to track a subject moving across the frame. The default 0 setting will allow for gradual AF point change. Selecting 1 or 2 will gradually increase the speed at which a different AF point is selected. In the latest cameras, including the EOS-1D X Mark III, EOS R5 and EOS R6, the AF point auto-switching options have moved to the general AF menus.

AF configuration presets

While the configuration controls can be set to custom values, there are up to six presets designed for different scenarios and, instead of having to remember what each setting does, the camera provides an icon and example usage within the menu display to make selecting the correct option easy. The most recent high-end EOS cameras also feature an Auto setting in which the camera adjusts the tracking automatically as it adapts to the subject movement.

Case 1: a versatile multi-purpose setting – The default setting, Case 1 is for general-purpose shooting. It provides accurate and fast focusing across a wide range of shooting situations. However, simply selecting this option for everything will mean you don't make full use of the AF system, and a little adjustment will most likely give you even better results.

Case 2: the camera continues to track subjects, ignoring possible obstacles – The camera will continue to track focus the subject, even if the subject moves away from the AF point or an obstacle momentarily comes between you and your subject. This is useful for subjects such as swimming, freestyle skiing or tennis.

Case 3: the camera focuses instantly when a subject enters the active AF area – Case 3 is useful for rapidly locking on to a new subject, or for switching between subjects rapidly. As an example, this would suit alpine skiing or the start of a cycle race, where there are several subjects and you may wish to select between them quickly.

Case 4: for subjects that accelerate or decelerate quickly – Case 4 is designed for subjects that change speed or direction rapidly, as happens in motorsports or football. The camera will prioritise the speed of tracking to keep up with these changes in speed, even if the focus results suggest it is a very rapid change in focus distance.

Case 5: for subjects that move erratically in any direction – Case 5 is designed for use with automatic AF point selection, Zone AF and AF Point Expansion and subjects that move erratically, up and down or left and right. The settings allow the camera to switch AF points rapidly to keep track of the motion. It is most suited to subjects like figure skaters or aerobatic flying displays, where erratic motion is likely to be encountered.

Case 6: for subjects that change speed and move erratically – Case 6 is like a combination of both Case 4 and Case 5. Like Case 5 it is used with Automatic AF point selection, Zone AF and AF Point Expansion. Even if the subject starts or stops suddenly or makes erratic direction changes, this setting will enable the camera to respond quickly to keep the focus accurately tracked on the subject. This setting is most useful when shooting subjects like basketball or gymnastics or birds in flight, where abrupt speed and direction changes are common.

Case A: the tracking adapts automatically to subject movement – Case A is a more advanced default setting than Case 1 as the camera automatically adapts to the subject's movement and the parameters are adjusted automatically. This was introduced in the EOS-1D X Mark III and is also available in the EOS R5 and EOS R6. These models and future cameras no longer offer Case 5 and Case 6.
A fast-moving red and white racing car in sharp focus against a blurred background and foreground of greenery.

Unless you have a completely clear view when you're panning the camera to follow a moving subject, objects such as trees, pillars and street lights are likely to come briefly between the camera and the subject. Selecting Case 2 in the AF configuration controls screen instructs the camera to ignore these objects and keep tracking the intended subject. © Frits Van Eldik

The AF configuration screen of an EOS R6 showing the camera set to Servo AF with the Case 2 preset selected.

AF Microadjustment

Some EOS DSLRs feature AF Microadjustment that allows you to move the exact point of focus slightly forwards or backwards to ensure that the camera and lens are in perfect alignment.

Because of the increase in resolution of camera sensors, any slight focus misalignment is more visible when reviewing images. Although the cameras and lenses are precision-engineered, there is a tolerance range and, in some cases, the camera could be at one end of the range and the lens at the other. In this instance, you would notice the focus point is either in front of where you thought it should be, or behind. Microadjustment enables you to correct for this.

This option is not required with mirrorless cameras as the imaging sensor is also the focusing sensor.

When AF Microadjustment is selected, the camera detects the serial number of the lens so you can make an adjustment for each specific lens based on serial number. If the serial number of your lens isn't detected, it's possible to register a serial number for a lens within the camera menu.

It's also possible to make AF Microadjustments with zoom lenses. In the past it was possible to register only one microadjustment setting for each lens. However, with more modern systems, it's possible to make adjustments for both the wide-angle and telephoto settings of a zoom lens.

Chromatic aberration correction

Because each wavelength of light is refracted differently, there can be some chromatic aberration caused by the splitting of light into two phase when light passes through the autofocus optics of a DSLR in viewfinder mode.

The errors caused by the chromatic aberration can be compensated for as part of the autofocus algorithm performed in the AF processor. However, since different types of light exhibit different amounts of chromatic aberration, it is important to know what light conditions you are shooting under to perform the correct adjustment.

The Dual-layer Metering Sensor design solves this problem. By having two metering layers sensitive to different colours of light, the camera can determine how much red/green or blue/green light is in the scene. With this information, the AF processor can accurately adjust for any chromatic aberrations that may occur in the AF system. This is useful in all conditions but is especially effective when you're shooting in low light conditions or under artificial lighting.
A woman's eye at 5x magnification in the Live View display on the back of a Canon EOS 6D Mark II.

To check focus (when the AF method is not Tracking), you can magnify the display by approximately either 5x or 10x by pressing the magnifying glass button or tapping the icon on the display.

A view from a high angle of a dirt bike track with two riders racing each other.

This tight framing from a high angle would make it difficult to predict where a subject will appear in the frame. This means that a large AF area such as Large Zone AF: Vertical and Case 3 would be a good choice as the camera will respond quickly when the subject enters the AF frame. © Richard Walch

Orientation linked AF point

One issue with cameras that have multiple AF points is that the active AF point may need to change to keep the subject sharp when you switch from shooting in landscape to shooting in portrait orientation. For example, if your active AF point is at the top-left in landscape orientation, where a face is likely to be, but you then turn the camera sideways, then that point is now at the bottom-left, where faces are less likely to occur.

Some EOS cameras enable this switch in AF point or zone mode to be made automatically if the option is selected via the Custom Function menu.

Registered AF point

Some EOS cameras feature a Registered AF Point (also known as the Home Position). This allows you to pre-select a focus point and switch to it instantly whenever required by pressing a button selected for the task in the Custom Controls section of the menu. This may work in combination with the orientation linking function, enabling you to register a point for each orientation.

This feature is most useful for sports photographers, who may have two or three areas of the viewfinder where the action is most likely to take place. However, remembering to switch focus points at the right moment will need some practice.

On some cameras, a specific AF method can be saved for each Registered AF Point, so that the camera switches to the preset AF method when you switch focus point. This can be very useful for a bird photographer, for example, who could use this feature to quickly switch from a Spot AF point for precisely focusing on a static subject, to a wide Zone AF pattern for quickly acquiring and tracking a bird in flight.

If your camera does not offer a Registered AF point, switch to the centre point instead, and use focus lock to keep your chosen subject sharp.

Focus lock

The central AF point of a DSLR in viewfinder mode is usually the most sensitive, which is helpful with tricky subjects or in low light. While using a single central focusing point might seem limiting because you don't always want your subject to be in the centre of the frame, it can actually be very versatile.

To focus on an off-centre subject, use the focus-and-recompose technique:

  1. Move the camera, positioning the centre of the viewfinder frame and the active AF point over the centre of the subject.
  2. Partially press the shutter button to lock the focus without taking a picture.
  3. Keep the button partially pressed and move the camera to recompose the viewfinder image.
  4. Press the shutter button right down to take the picture.

Focus lock also locks the exposure. If you want to take focus and exposure readings from different areas, first compose the image in the viewfinder so that the area you want to take an exposure reading from is in the centre of the frame. Then press the exposure lock (*) button on the back of the camera. Next, recompose the image and press the shutter button. This will refocus the lens and take the picture, while using the locked exposure.

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