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Infobank

Autofocus (AF)

How AF works

Autofocus is so easy to use that the focus switch on some of your lenses may never have been moved to the manual position.

Most of the time autofocusing gives excellent results. It is faster than manual focusing, and often more accurate. However, autofocusing operates by a series of rules (algorithms). There may be times when these do not give the results you want.

EOS cameras use a highly sensitive line sensor for autofocus ranging. Called BASIS (BAse Stored Image Sensor), it consists of two 48-bit line sensors and associated amplifier circuitry.

The sensor is in the base of the camera. A sub-mirror behind the camera’s reflex mirror reflects light down to the sensor. 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 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.

Focus sensors

Multiple sensors

The first EOS cameras used a single AF sensor. This sensor was positioned to focus the lens on the subject in the centre of the viewfinder image.

However, there will be many circumstances where the main subject is not in the centre of the frame. This is why most EOS models 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 subject. The camera analyses these readings and then decides which focusing point to activate. The lens is focused using the information from this active point.

Most EOS professional digital cameras use a 45-point autofocus sensor.

Black: Vertical line sensors operational with lenses having an f5.6 or greater aperture.

Blue: Cross-type sensors operational with lenses having an f2.8 or greater aperture
Vertical line sensors operational with lenses having an f5.6 or greater aperture.

Red: Cross-type sensor operational with lenses having an f4 of greater aperture
Vertical line sensor operational with lenses having an effective aperture of f8 or greater.

What you may notice looking at this is that of the seven cross type sensors, six of them (the blue ones) are only functional with a small number of lenses, mainly the fast prime lenses and the f2.8 zoom lenses. If you are not using one of those lenses, then those cross-type sensors perform no function in autofocusing your lens.

EOS-1D X AF System

The EOS-1D X DSLR features a completely new autofocus system that's been designed to provide consistent focusing performance, no matter what the subject is or how bad the lighting conditions happen to be.

To achieve the goal of optimum AF in diverse situations and lighting conditions, the EOS-1D X makes use of far more than just an autofocus sensor, as previous EOS cameras have done. Instead data is collected from the 61-point AF sensor, the auto exposure sensors, an AF correction light-source detection sensor and, with certain lenses, a panning detection gyro sensor. While these sensors provide a benefit to One-Shot AF shooting, the major benefit is found with AI Servo, where they can help identify the subject by not only contrast, but also colour. To make AF set-up easier the EOS-1D X also features a simplified AF settings menu where a description and example uses for each function are displayed.

 

The AF sensor of the EOS-1D X DSLR. The camera has a high-precision AF system with 61 user selectable AF points, 41 of which are cross-type points, and five dual cross-type points for extra precision.

The AF sensor features 61 AF points. Of these 61 AF points, up to 41 of them will function as cross-type sensors that are able to detect contrast both horizontally and vertically. The exact number of cross-type sensors will vary depending on the lens you are using. The spread of AF points is roughly similar to that on the EOS-1D Mark IV but with far more AF points the precision is greater, especially when tracking moving subjects.

To improve focus accuracy, all 61 AF points feature a dual-line zigzag arrangement, as seen on three AF points within the EOS 7D. This arrangement provides the best aspects of both increased pixel pitch for finer precision and increased AF tracking speed with extra data points, without any of the drawbacks of either solution alone, allowing for both fast and accurate AF.

Five of the central AF points, arranged vertically down the mid-line of the frame, function as dual-cross type AF points with lenses featuring an f/2.8 or faster maximum aperture (as seen on the central AF point of the EOS 7D). This means they are also arranged with a diagonally orientated AF point in an ‘X', plus a conventional horizontally and vertically arranged AF point, like a ‘+', offering increased focus precision.

With lenses that have a maximum aperture of f/5.6, or faster, the central bank of 21 AF points will all function as cross-type AF sensors, and the left and right banks of 20 AF points each will act as cross-type sensors at f/4 and f/5.6.

An advantage of the increased focus sensitivity is the ability to detect extreme defocus and correct accordingly. By using the whole AF sensor, where every point is vertical line sensitive at f/5.6 or greater, the lens can be refocused much more quickly than before. Additionally, the EOS-1D X can lower light levels than previous models. Using a single central AF point with an f/2.8 lens, the EOS-1D X is able to focus in EV -2, which is the equivalent of shooting under the light of the full moon.

 

The AF point selection menu screen of the EOS-1D X DSLR.

With a predictive AF system the camera is continuously recording the position of the subject and predicting 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 III algorithm will ignore the negative result and the next focus point is based on the previous accurate results. Equally, the EOS-1D X will 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. Equally, if there is suddenly a 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 results.

The increased sensitivity of the focus system has also allowed for faster predictive focus measurements. In previous EOS cameras there was a warm-up period while the AF system began tracking. This has now been reduced so that the EOS-1D X can begin predictive tracking as soon as a subject begins to move.

Light source detection

First seen in the EOS 7D, the EOS-1D X features a light source detection system to improve focus accuracy under artificial lighting. Because of the higher resolution of the AF sensors, chromatic aberration within the optics could lead to focus errors because the different colours of light waves are focused at slightly different distances. However, because the camera is able to determine how much red/green or blue/green light there is in a scene, the AF system can adjust for any potential chromatic aberrations that may occur within the AF system.

Focusing during panning

The EOS-1D X features a built-in 2-axis gyro sensor that can detect camera motion both horizontally and vertically. If panning motion is detected, it will stop the AF point switching to a new subject should you pan across an obstacle within the frame during shooting.

When using the Mark II super telephoto lenses, such as the EF300mm f/2.8L IS II USM or the EF400mm f/2.8L IS II USM, the camera will use the IS gyro sensor located in the lens, rather than the one located in the camera.

EOS intelligent Tracking and Recognition AF (EOS iTR AF)

In addition to all the data provided by the AF system, the camera will also make use of data provided by the 100,000-pixel RGB AE sensor and AE DIGIC 4 processor to improve focus tracking in AI Servo mode.

By default, AF systems operate based on contrast detection. They 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. Since the AE system of the EOS-1D X can detect the colour of a subject, this information can be passed to the AF system to improve the tracking.

By using the colour of the subject that was initially focused, the AF system can track the movement of that subject, both by contrast and 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 by manually selecting an AF point you can ensure the correct face is focused initially and then tracked in subsequent frames.

 

The AF unit of the EOS-1D X DSLR.

AF system durability

To ensure this system is durable enough for professional use, the materials used have been chosen to withstand high and low temperatures equally well, as well as conditions of high humidity. The sub-mirror of the AF system has also been modified from an elliptical shape, as found in the EOS-1D Mark IV, to a flat surface in order to provide higher AF stability.

AF point selection modes

First found on the EOS 7D, the EOS-1D X features six different AF point selection modes, including Spot AF for all lenses.

In Single Point AF a single AF point, from the 61 available, is manually selected and used by the camera for focusing. With Auto Selection, the camera will select from any of the 61 AF points available to focus the subject. Single Point Spot AF is the same as Single Point AF, but in Spot focusing, 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 a lion lying in long grass. However, Spot AF is not recommended for fast moving subjects or in very low light conditions. When 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.

 

An AF menu screen of the EOS-1D X showing manually selected Spot AF options.

For more control over tracking moving subjects, there are two AF point expansion settings. In AF point expansion, a single AF point is manually selected and the camera will then use four surrounding points to help to track the subject. The options are either a cross arrangement, with the points immediately above, below, left and right being used, or the surrounding eight points around your manually selected point.

The final AF selection mode is Zone AF. As on the EOS 7D, this allows you to select one of nine zones. Within the selected zone, the AF points will be chosen by the camera in the same manner as with automatic AF point selection, but the points the camera can select from are restricted to your chosen zone.

AF menu tab

For easier menu navigation and setting, all the AF settings and Custom Functions are now grouped into one menu tab, so there is no need to jump into different menu areas to make changes.

AF Configuration Tool

Within the AF settings there are also some new configuration options. The first of these is Acceleration/Deceleration tracking. This is useful for subjects that change speed, like a racing car. 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. Settings 1 and 2 are designed for subjects that move suddenly or that accelerate or stop suddenly. They should not be used with smooth moving subjects as it could make the focus more unstable for those subjects.

The second configuration option is for AF point auto-switching. This 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.

There are 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 default setting ‘Case 1' is for general purpose shooting. It will provide accurate and fast focus 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, with a little adjustment, you will most likely achieve even better results.

‘Case 2' is designed for situations where the subject may move away from the AF point momentarily. 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' will allow you to instantly focus on subjects that enter the AF point area. It is useful for rapidly locking onto 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' 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' 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' is like a combination of both ‘Case 4' and ‘Case 5' and is for subjects that change speed abruptly and move erratically. 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, where speed and direction changes are common.

AF Microadjustment

AF Microadjustment 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 mis-alignment is more visible when reviewing images. Although the cameras and lenses are made to extremely high tolerances 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, or behind where you thought it should be. Microadjustment allows you to correct for this.

 

Menu screen showing input of the serial number of a lens. To aid the AF Microadjustment of the EOS-1D X serial numbers of individual lenses can be input.

The AF Microadjustment in the EOS-1D X is able to detect a lens' serial number so you can make an adjustment by 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.

Microadjustment with zoom lenses is also more comprehensive. In the past it was only possible to register one microadjustment setting for each lens. However, with the EOS-1D X it's possible to make adjustments for both the wideangle and telephoto settings of a zoom lens.

EOS 5D Mark III AF system

The EOS 5D Mark III takes many of the features of the EOS-1D X and brings them to the EOS 5D range. The focus system in the EOS 5D Mark III is exactly the same as that found in the EOS-1D X, with one difference – where the EOS-1D X can make use of the 100,000-pixel RGB AE sensor and dedicated DIGIC 4 processor for Colour and Face tracking (EOS iTR AF) the EOS 5D Mark III does not have the same AE sensor and therefore cannot perform colour or face tracking.

EOS-1D Mark IV AF system

The EOS-1D Mark IV features a high-speed, high-precision sensor with 45 user-selectable focus points of which 39 are cross-type. Compared to the EOS-1D Mark III which featured 19 cross-type AF points, the EOS-1D Mark IV offers more flexibility in composition, especially when tracking moving subjects.

As with all EOS models, the central AF point is the most sensitive. It has a cross-type sensor that is vertical line sensitive at f/2.8 to f/4 and horizontal line sensitive from f/5.6 to f/8. With any lens that features a maximum aperture of f/4 or faster, high precision cross-type AF is available. With a lens or lens-extender combination that has a maximum aperture of f/8 or faster, autofocus will still function at the central point using horizontal line detection.

The remaining 38 cross-type AF points are vertical line sensitive at f/2.8 and horizontal line sensitive at f/5.6. If the lens in use has a maximum aperture of f/2.8 or faster, high precision focusing with cross-type sensors will be possible. If the lens’ maximum aperture if f/5.6 or faster, AF will still function with horizontal line detection.

In an upgrade from previous EOS cameras, the following lenses and lens/extender combinations will also function with cross-type AF at all 39 cross-type AF points, even though the maximum aperture is f/4:

  • EF17-40mm f/4L USM
  • EF24-105mm f/4L IS USM
  • EF70-200mm f/2.8L IS USM + Extender EF1.4x II
  • EF200mm f/2L IS USM + Extender EF2x II
  • EF300mm f/2.8L IS USM + Extender EF1.4xII
  • EF400mm f/2.8L IS USM + Extender EF1.4x II

In automatic autofocus point selection all 45 points will be used and the camera will select the AF point automatically to suit the shooting conditions. In manual AF point selection you can manually select any of the 19 cross-type AF points. Additionally, by using CFn. III-10 you can choose to limit the number of AF points that are manually selectable from a choice of using all 45, 19, 11, 9 inner points or 9 outer points.

 

AF point selection allows you to select from 19 cross-type AF points. In Cfn. III-10 manual AF point selection means all 45 AF points can be selected or you can choose to limit the number of AF points as per your requirements.

If you use manual AF point selection, the six AF points that are not cross-type sensitive will function as horizontal line sensors with lenses featuring a maximum aperture of f/5.6 or faster. They have the same performance as the EOS-1D Mark III assist points working with f/5.6 or faster lenses.

The AF points immediately above and below the central AF point feature improved performance and precision due to the use of two sensor lines from which to detect focus.

As part of the improvements in sensitivity at the central AF point, the camera can detect the subject even when it is completely de-focused. This has the effect of reducing the lens hunting while trying to detect the subject with the effect that the AF system functions faster and is easier to control.

Like lenses, the autofocus optical system can be subject to some form of chromatic aberration. Under some lighting situations, most notably artificial light sources, this can lead to focus inaccuracies. The EOS-1D Mark IV features a dedicated light source detection sensor at the back of the pentaprism. This allows the camera to determine how much, if any, aberration there will be in the autofocus optics and correct appropriately for it. This results in more consistent autofocus performance.

Like all EOS-1 series models, and the EOS 7D, the EOS-1D Mark IV features a dedicated autofocus microprocessor for handling AF calculations. To cope with AI Servo running at 10fps, the autofocus processor shares this processing with the camera processor, thereby improving the speed of AF calculations.

The autofocus algorithm has also been adjusted based on user feedback. The EOS-1D Mark IV has a more balanced response that has been tuned to offer a more stable autofocus operation.

AI Servo II autofocus functions in a similar way to the system found in the EOS 7D – when tracking a moving subject it will predict where the subject should be based on current trajectory and speed. If the camera obtains an autofocus reading that is substantially different to what is expected, it will ignore that reading and continue to the next predicted position. This also works effectively when an obstacle passes in front of the main subject. In this instance, the camera will continue to track the main subject basing the focus on the results calculated immediately before the obstacle passed between the camera and the subject. The changes in the autofocus system and algorithm mean that AI Servo II AF can now be used accurately with close-up subjects as well.

To enhance this stability, the camera uses a modified lens drive. If there is a sudden change in the autofocus results from the sensor, the camera will not jump to the new focus distance because it is likely to have been caused by erratic subject movement rather than a deliberate change by the photographer. In this instance, the camera will not jump to the new focus distance immediately. Instead, it will slowly drive then lens towards the new focus distance. This means that as soon as you put the AF point back onto the subject, the lens is unlikely to be a long way out of focus and autofocus will be able to continue accurately.

Like the EOS 7D, the EOS-1D Mark IV features an immediate response to subject movement. With previous EOS models, the camera required time to lock onto the subject and track it accurately. With the EOS-1D Mark IV, this is not the case – as soon as subject movement is detected, it will start to track accurately and efficiently. This improves the chance of capturing rapid movement, as there is less need to wait for the autofocus system to lock onto the subject.

Professional photographers work in many different genres and in many different ways. To suit this diverse group of working methods, the autofocus system features a variety of custom functions to allow the camera to be tailored exactly to the situation. In the same way a racing car will perform at its best when tuned to suit the track, so the EOS-1D Mark IV will provide the best results when set up to suit the situation.

 

The EOS-1D Mark IV offers various AF mode options, including Spot AF to help to deliver more accurate AF.

Spot AF is a new AF mode first seen on the EOS 7D. On the EOS-1D Mark IV it is available with any of the super-telephoto lenses that feature an AF Stop button. C.Fn III-6-7 activates Spot AF when the lens AF stop button is pressed. This has the effect of reducing the size of area used to perform autofocus to a smaller point. The result is more accurate AF less likely to be influenced by contrast away from the edge of the AF point. Spot AF is not provided as a standard autofocus mode because when tracking moving subjects it may slow the autofocus operation as it uses a smaller area to detect focus from, and is therefore slower to detect defocus and respond to it.

When tracking erratically moving subjects, it can be difficult to keep a single AF point on the subject. In this situation you may find that autofocus point expansion, C.Fn III-8, is a more reliable focusing method to use because if the manually selected AF point cannot achieve focus, the surrounding AF assist points will be used. C.Fn III-8 provides

If you select option 3, all 45 points, the expansion AF points will follow the selected AF point. If the AF mode is set to AI Servo, then up to 18 AF points surrounding the active AF point will be used. Each time the subject moves to a different AF point, the surrounding expanded AF points will also move to follow. As the subject moves around the frame, the selected AF point will also follow to track – this can be seen by the AF point being lit in the viewfinder. It is similar to using automatic AF point selection, but the AF area is concentrated to a smaller area and it is therefore easier to select a specific area to focus on. It also allows you to choose the AF point to start tracking from. Compared to automatic AF point selection, or the function of the EOS-1D Mark III, where autofocus searching will begin from the central point first, this gives more control over the composition.

AF point expansion will also work in One Shot AF. The camera will first start searching with six, then 18, then all remaining 44 points until focus is achieved. Many users will like the flexibility of 45 selectable AF points. However, in some situations it can be beneficial to limit the number of selectable AF points to speed up the selection process. C.Fn III-10 allows you to limit the number of focus points from 45 to 19 (the same as in the EOS-1D Mark III), 9 inner points, 9 outer points or 11 points (as found on the EOS-1D Mark II/N).

AF point expansion will also work in One Shot AF. The camera will first start searching with six, then 18, then all remaining 44 points until focus is achieved. Many users will like the flexibility of 45 selectable AF points. However, in some situations it can be beneficial to limit the number of selectable AF points to speed up the selection process. C.Fn III-10 allows you to limit the number of focus points from 45 to 19 (the same as in the EOS-1D Mark III), 9 inner points, 9 outer points or 11 points (as found on the EOS-1D Mark II/N).

 

Via custom function C.Fn III-16 the EOS-1D Mark IV offers orientation linked AF points for swift shooting even when rotating the camera from horizontal to vertical orientations.

Another feature first found on the EOS 7D, that has been incorporated into the EOS-1D Mark IV, is the orientation aware autofocus system. Using C.Fn III-16 you can store three different AF configurations that are automatically switched to depending on the camera orientation – horizontal, grip up or grip down. In many situations you may wish to change autofocus point when you rotate the camera from a horizontal to vertical orientation to ensure you keep focus on the same point. By using this custom function this can be achieved without you having to press a button or select a different AF point manually. It speeds up your working with the system and will mean that you are less likely to miss shots through having to select a different AF point.

Like the EOS-1D Mark III, the EOS-1D Mark IV offers AF Microadjustment so you can tailor the calibration of your cameras and lenses for the most accurate results. The range of adjustment is +/- 20 steps. This microadjustment only affects the AF sensor and not the autofocus performance of Live Mode and Face detection AF in Live View Mode as these use contrast based detection from the image sensor.

EOS 7D AF system

The EOS 7D features a 19-point autofocus system where all AF points are cross-type sensitive and AF calculations are handled by a dedicated AF processor for a faster, more accurate response.

The sensor in the EOS 7D has the same coverage area as the sensor found in the EOS 50D, but it has 10 extra points making it a 19-point system with several new AF point selection methods.

When designing the sensor, a larger size was considered however research has shown that when all factors are considered, a larger AF sensor is not necessarily better. Larger sensors are more susceptible to changes in temperature with the result that they change size getting either larger or smaller as the temperature rises and falls. This in turn has an impact on the accuracy of the AF system overall. In order to maintain consistency and accuracy the size of the sensor has been optimised to ensure that changes in temperature have little or no affect on AF performance.

Autofocus sensors in an AF system detect lines of contrast. They are either sensitive to vertical lines or horizontal lines. However, in real-world usage having sensors only sensitive to one type of line orientation is not the most effective solution. As such, all AF points in the EOS 7D are cross-type sensors with lenses which have a maximum aperture of f/5.6 or faster.

For enhanced precision, the central AF point features extra sensitivity when used with lenses having a maximum aperture of f/2.8 or faster thanks to the diagonal arrangement of the cross-type sensor. This diagonal placement with the sensors forming an X-arrangement means that the f/2.8 sensors do not overlap with the f/5.6 sensors at the same point as they have a horizontal/vertical arrangement. This avoids any interference between the two sensors and offers both fast tracking performance from the f/5.6 sensor and extra precision from the f/2.8 sensor.

 

The EOS 7D’s AF sensor.

f/5.6 sensors are especially good at predictive tracking focus, however, the designers wanted to find a solution that also offers high focus accuracy as well. Increasing focus accuracy can be achieved by using a higher resolution focus sensor however, simply increasing the pitch, and therefore resolution, of a single line too much leads to a lower signal to noise ratio in terms of light levels recorded. To combat this, the centre top, centre middle and centre bottom sensors (when the camera is held in a landscape orientation) have a dual zigzag horizontal line sensor. The dual zigzag arrangement solves this problem by using two focus sensors that are slightly offset from each other. By performing AF calculations based on the two lines sampling slightly different areas, the focusing resolution is improved leading to a more precise focus result while still retaining the responsive AF tracking.

 

Diagram of the cross-type sensor configuration

Autofocus accuracy

The EOS 7D also corrects for chromatic aberration caused within the autofocus optics. When light passes through the autofocus optics are required to split the image into two phases and, as a result, chromatic aberrations can occur. This is because each wavelength of light is refracted differently.

These 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 photographing 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 shooting in low light or under artificial lighting conditions.

Autofocus Point Selection Method

The EOS 7D has a redesigned AF point selection methodology to make use of the 19 AF points.

This provides five options for AF point selection.

  • 1. Manual Selection: Single Point AF – any of the 19 AF points can be selected giving you flexibility in framing your subjects when using the creative shooting modes of P, Tv, Av, M and Bulb. In Green Square mode or the Creative Auto mode the central AF point will be used.
  • 2. Manual selection: Spot AF – On a normal autofocus sensor, the sensor for each point is actually larger than the AF point shown in the viewfinder. Spot AF uses the same AF point to perform focus but the area it uses is much smaller – only fractionally larger than the AF point displayed in the viewfinder. This mode is useful in situations where you want to be very precise about the area you focus on – when shooting small macro subjects or when taking close portraits.

    In the portrait situation it allows you to focus on the eye of your model with a much reduced chance that the edge of the AF point will actually find focus on an eyelash or eyebrow with the result when shooting with very shallow depth-of-field that the image is slightly out of focus. This mode can be used when shooting moving subjects in AI Servo, though it will require very good reactions and subject tracking skills from the photographer to keep the smaller AF point exactly where it needs to be on the subject.

    Because of the smaller area used to perform focusing on the AF sensor, the Spot AF mode is not as fast at detecting and correcting extreme defocus situations.

  • 3. Manual selection: AF point expansion – focus is achieved with the selected AF point with assistance provided by the surrounding AF points. This is especially useful when tracking fast moving subjects and was a custom function on previous EOS cameras.
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    Examples of expanded AF points.

  • 4. Manual selection: Zone AF – the area used to focus with is larger with the user able to select one of five zones (left hand side, top, bottom, centre and right hand side) to use at any one time. This makes it easier to track moving subjects, especially those who may be off-centre in your composition.
  • 5. Auto Selection: 19 point AF – When using autofocus point selection in One Shot AF, the closest subject to the camera is used to achieve focus. Once focus is achieved, the AF point or points achieving focus will illuminate. In AI Servo AF, the AF point that starts the AF tracking can be selected. On all previous EOS models, the centre point was always used as the primary AF point to first achieve focus. By being able to select the AF point to start tracking with you have a much greater flexibility in how you compose your images.

EOS 5D Mark II AF system

The EOS 5D and EOS 5D Mark II have a 9-point AF unit with six supplemental AF points. This system improves subject detection and focusing precision at the centre, the most frequently used area, and significantly enhances subject tracking performance.

The nine AF points are concentrated at the centre, but the extreme left and right AF points are located in the same positions as the corresponding AF points on the EOS 1Ds Mark II.

In addition, the six supplemental AF points, which are grouped around the centre of the image, provide highly accurate focusing and do an especially good job of tracking a moving subject in the AI Servo AF mode.

These supplemental AF points are not visible in the finder and cannot be selected manually, but they can be seen in the ZoomBrowser or ImageBrowser software. They are used during automatic AF point selection (AF calculation speed and accuracy are as good as, or better than, the EOS 30D) and as AF area expansion points when the centre AF point is selected in C.Fn-17-1. They do not function in the One-Shot AF mode.

The centre AF point has a special hybrid design. With f/2.8 or faster lenses, focusing is a two-step process. First, the f/5.6-sensitive cross-type sensor components are used to focus. When focus is almost achieved, a switch is made to the f/2.8-sensitive vertical line-sensitive sensors for high-precision focusing.

Additionally, the centre AF point’s f/5.6 sensors each have two lines with individual pixels arranged in a zigzag pattern, making a total of four lines for horizontal line-sensitive focusing at the centre.

With this improvement, the centre AF point can do a better job of detecting difficult-to-read, low-contrast subjects, improving the overall performance of the autofocusing system.

All six supplemental AF points are f/5.6- sensitive. The two supplemental AF points directly above and below the centre are also f/2.8-sensitive when a lens of f/2.8 or brighter is used.

Focus lock

Cameras with just a single central focusing point might appear to be limiting. After all, you don’t always want your subject to be in the centre of the frame. But these single sensor cameras are actually very versatile.

To focus on an off-centre subject, simply move the camera, positioning the centre of the viewfinder frame over the centre of the subject. Now partially press the shutter button. This will lock the focus without taking a picture. Keep the button partially pressed and move the camera to recompose the viewfinder image. Increase the pressure on the shutter button 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.

Registered AF point

A new feature found on some recent digital cameras is the Registered AF Point (which is also known as the Home Position). This allows you to pre-select a focus point and switch to it instantly whenever required.

On the EOS-1D and 1Ds series, you first select the AF point normally and then register it by pressing the Assist and FEL buttons.

During shooting you can return to this AF point at any time by pressing the Assist and AF point selection buttons together. Alternatively, by setting C.Fn.-18-1 on the EOS-1D or 1Ds models, you can switch to the registered AF point just by pressing the Assist button.

This feature is probably 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.

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.

Using multiple sensors

The camera can only use data from a single AF sensor to focus. When there is more than one focus point a selection has to be made. This can be done automatically or manually.

Automatic selection

If the camera is set for automatic selection of the focusing point, readings are taken from each point as the shutter button is partially pressed. If there are only three or five focusing points, the camera gives priority to the point covering the nearest part of the subject. This point becomes active and its data is used to focus the lens.

A single focusing point works well provided you can keep the subject in the centre of the frame.

Multi-point focusing means that the camera will automatically focus on off-centre subjects without the need for focus lock.

However, the more focusing points there are, the less chance the camera has of selecting the one you want. To overcome this problem, Canon introduced a new set of rules (algorithm) with 7-point focusing. Here, the camera does not only give priority to close subjects, it also compares the scene with data taken from a large number of sample photos and analyses the selection probability of each focusing point. The position of each point also affects the weighting given to its data. Focusing points higher up the scene are rated more important than those lower down.

An orientation sensor is built into the camera so that the system knows whether you are shooting with a horizontal or vertical format and can work out the order of the points down the scene.

If several focusing points end up with equal priority, the camera will make active the point covering the closer subject. The active point is visible in the viewfinder, either as a red highlight on the focusing screen or as an icon on the viewfinder read out.

Manual selection

Although automatic selection gives good results most of the time, there are situations where it will fail. If you frame a landscape with a tree branch, for example, the camera might focus on the branch rather than the more distant scene.

Manual selection allows you to choose the active focusing point before you take a picture. Although any one of the points can be selected, the centre point is often the most convenient. You can then operate the camera in the same way as a single-point model, taking the AF reading with the centre point and then using AF lock to hold the focus while you recompose the image. This way, you have full control over the result, rather than leaving important focusing decisions to the camera.

Evaluative metering

The metering system of your EOS camera is linked to the multipoint focusing. If an off-centre focusing point becomes active, the evaluative metering will also shift to give optimum exposure for the area covered by the point. Canon calls this ‘AIM’ − Advanced Integrated Multipoint control.

If you use the centre focusing point with AE lock, the exposure will be metered for the area covered by the centre point at the moment you activated AE lock. If this is not what you want, focus the lens manually so that you can use AE lock independently of the focus lock.

Autofocus modes

Most EOS cameras offer two or three different autofocusing modes. Although the end result is that the lens automatically focuses, it is best to set 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 the mode to use if you don’t know which mode to use. It is a good general purpose setting to suit most subjects. In practice, you compose your subject in the viewfinder and apply partial pressure to 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 view a different area of the subject. This gives a very quick and convenient method of achieving focus lock.

You can bring the main area of your subject to the centre of the viewfinder frame, apply partial pressure to the shutter button to focus, and then recompose the image in the viewfinder without the focus changing.

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 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 away with the focus lock of one-shot AF. 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.

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.

The AI Servo II AF mode that tracks moving subjects has been significantly improved in the EOS 7D.

Stable lens drive: The EOS 7D features a more stable lens drive. In other words, the AF algorithm has been improved to account for rapid changes in direction while not being too sensitive causing it to jump from one subject to another.

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 new algorithm will now ignore a reading if it is significantly different to what is expected based on other results. This helps to stop the lens jumping completely out of focus.

Secure focus tracking: Predicting the next position is based on the last known trajectory result immediately before the obstacle appears. This is a great benefit when shooting subjects like sport where one player may pass in front of the one you are tracking. This helps keep focus on a subject even if an obstacle passes between the camera and subject.

 

The EOS 7D AF tracking method screen.

Moderate lens drive: Although the AF algorithm will ignore obscure results when tracking a moving subject, it is quite likely that you may not keep the AF point exactly on the subject. If this happens and you do not immediately come back to the subject, instead of rapidly focusing on what appears to now be a new subject, the algorithm will only slowly move to the new values based on previous predictions of the next position of the subject. This means that should you drift away from the subject, the focus will not snap to the background immediately. It also helps if you then put the AF point back on the subject – by not being as far out of focus, the camera can re-find focus much more quickly.

Predictive control with quick response: On previous EOS models, the AF system requires a moment of time following the subject to ‘warm-up’ and track with maximum accuracy. The system on the EOS 7D no longer needs this ‘warm-up’ time and can track a subject immediately. This speeds up your focus lock and means you can find and track a moving subject quicker and more accurately.

AI Servo II AF can even be used when shooting macro subjects, as it can moderate its response accurately enough to work with the higher magnification levels.

Predictive focusing

With 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’. This is the brief amount of time between the pressing the button and the shutter actually opening. During this time, the reflex mirror inside the camera has to swing up to allow light passing through the lens to reach the film or digital sensor at the back of the camera.

Shutter lag on EOS cameras is very short − typically around 55ms (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 an hour 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 and hour. 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.

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.

AI Focus AF

AI Focus AF mode switches between the 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? Most photographers do not know − but many EOS cameras do.

If AI Focus AF is available and selected, the camera will automatically switch from One-shot AF to AI Servo AF mode when the camera detects subject movement of a certain speed. Practical tests suggest you need a fast-moving subject before the mode will change.

The camera detects movement by taking several AF readings as the shutter button is partially pressed. If the subject distance changes between readings, 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 you encounter a subject travelling at speed, you won’t have to remember to change the AF mode. However, most photographers shooting sport and wildlife prefer to set AI Servo AF.

Manual focusing

One focus mode neglected by many EOS users is manual. After all, why spend money on an autofocus camera and then switch the feature off?

Actually, manual focus is not a camera feature. It is set by a switch on the side of most EF lenses. Move the switch from ‘AF’ to ‘M’ and you will disable the autofocusing. Now you can focus by looking at the image in the viewfinder and turning the focusing ring on the lens.

There are a number of situations where autofocus might not give accurate results and manual focusing can be used instead:

  • Low contrast subjects (large areas of sea or sky, for example, or landscapes in mist)
  • Low light subjects (this is also a low contrast subject − outdoor pictures taken in the early evening or at night for example)
  • Subjects with bright reflections (water, snow and shiny metal can fool the AF system)

In addition, there are times when creative focusing is needed. Autofocusing tends to focus on the area of subject nearest to the camera. If you want to focus on a subject a little further away, you can centre this subject in the viewfinder, use One-shot AF to focus on it, and then switch the lens to manual focus so that there is no change after you recompose the image.

Manual focusing is often essential with extreme close-up photography, as you will rarely want to focus on the front of the subject, and the lens can struggle to find focus.

Full-time manual focusing

Although the EOS focusing system is designed to be fully automatic, many USM lenses allow you to focus manually immediately after One-shot AF without the need to switch the lens to manual focus. This is called full-time manual focusing (FTMF). It is especially useful with telephoto lenses, where you might want to fine-tune the focus before taking a picture.

Some of the early lenses with this feature use electronic FTMF, but more recent lenses use a mechanical system. There is no difference in use. In One-shot AF mode, apply partial pressure to the shutter button to activate the autofocus. Then, keeping partial pressure on the button, turn the lens focusing ring to fine-tune the focus. Apply full pressure to the button to take the picture. If you remove your finger from the shutter button after FTMF, the lens will refocus the next time you partially press the button, losing the manual fine-tuning.

If you find it difficult to keep partial pressure on the shutter button without accidentally firing the shutter, check to see if your camera allows you to set Custom Function 4-1. This transfers the autofocus activation to the star button on the rear of the camera, reducing the risk of accidental exposures.

Here is a list of some EF lenses that offer full-time manual focus.

  • EF14mm f/2.8L USM
  • EF20mm f/2.8 USM
  • EF24mm f/1.4L USM
  • EF28mm f/1.8 USM
  • EF35mm f/1.4L USM
  • EF50mm f/1.0L USM
  • EF50mm f/1.4 USM
  • EF-S60mm f/2.8 Macro USM
  • EF85mm f/1.2L USM
  • EF85mm f/1.2L II USM
  • EF85mm f/1.8 USM
  • EF100mm f/2 USM
  • EF100mm f/2.8 Macro USM
  • EF135mm f/2L USM
  • EF180mm f/3.5L Macro USM
  • EF200mm f/1.8L USM
  • EF200mm f/2L IS USM
  • EF200mm f/2.8L USM
  • EF200mm f/2.8L II USM
  • EF300mm f/2.8L USM
  • EF300mm f/2.8L IS USM
  • EF300mm f/4L USM
  • EF300mm f/4L IS USM
  • EF400mm f/2.8L USM
  • EF400mm f/2.8L USM II
  • EF400mm f/2.8L IS USM
  • EF400mm f/4 DO IS USM
  • EF400mm f/5.6L USM
  • EF500mm f/4L IS USM
  • EF500mm f/4.5L USM
  • EF600mm f/4L USM
  • EF600mm f/4L IS USM
  • EF800mm f/5.6L IS USM
  • EF1200mm f/5.6L USM
  • EF-S10-22mm f/3.5-4.5 USM
  • EF16-35mm f/2.8L USM
  • EF17-35mm f/2.8L USM
  • EF17-40mm f/4L USM
  • EF-S17-55mm f/2.8 IS USM
  • EF-S17-85mm f/4-5.6 IS USM
  • EF20-35mm f/3.5-4.5 USM
  • EF24-70mm f/2.8L USM
  • EF24-85mm f/3.5-4.5 USM
  • EF24-105mm f/4L IS USM
  • EF24-85mm f/3.5-4.5 USM
  • EF28-70mm f/2.8L USM
  • EF28-80mm f/2.8-4L USM
  • EF28-80mm f/3.5-5.6 USM
  • EF28-80mm f/3.5-5.6 II USM
  • EF28-105mm f/3.5-4.5 USM
  • EF28-105mm f/3.5-4.5 II USM
  • EF28-135mm f/3.5-5.6 IS USM
  • EF28-300mm f/3.5-5.6L IS USM
  • EF35-135mm f/4-5.6 USM
  • EF35-350mm f/3.5-5.6L USM
  • EF70-200mm f/2.8L USM
  • EF70-200mm f/2.8L IS USM
  • EF70-200mm f/4L USM
  • EF70-210mm f/3.5-4.5 USM
  • EF70-300mm f/4.5-5.6 DO IS USM
  • EF100-300mm f/4.5-5.6 USM
  • EF100-400mm f/4.5-5.6L IS USM