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.

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.