Canon Professional Network

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 5D AF system

The EOS 5D has 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.

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.

Autofocus (AF)