Making the most of AF technologies.
The better you understand your camera’s focusing system, the better you’ll be able to achieve the outcomes you expect, so it pays to understand how it operates. Autofocusing typically operates in the following sequence:
- Half-pressing the shutter button causes the AF processor to make a small change in the focus of the lens.
- The AF processor checks the AF sensor(s) to find out whether and by how much focus has improved.
- Using this information, the processor triggers the drive mechanism to set the lens to a new focusing distance.
These three steps are repeated until satisfactory sharpness is achieved. The process usually takes a fraction of a second, although in some situations the camera will drive the lens elements back and forth, ‘hunting’ for focus.
AF System Components
All autofocusing systems contain three main components:
- An array of detectors (sensors) that pick up contrast signals from the scene and pass this information to a microprocessor for analysis.
- One or more lens elements that are moved to provide optimal sharpness at the detected point(s).
- Drive motors that move those lens elements.
Weaknesses in any of these components can compromise autofocusing. Camera manufacturers also make some compromises in system designs to reduce production costs and differentiate between cameras at different levels.
AF Sensor Points
Although the viewfinder doesn’t indicate what types of sensors a camera has, it usually displays the number of AF points available. Most cameras also show which points are being used when you half-press the shutter button.
Photographers can choose from at least three AF area modes. Single-point lets you select a single focus point from anywhere in the AF point array. The dynamic area, AF point expansion or Zone AF mode (the terminology varies with different manufacturers) enlarges the area to include a group of surrounding points.
Tracking AF keeps the subject in focus as it moves across the field of view, taking data from each sensor the subject passes. Some cameras also provide an auto-area AF mode in which the camera automatically detects the subject and selects the AF points.
Focus confirmation indicators can be as simple as a pinpoint light showing the active sensor point or outlining of the active AF area, as shown in this illustration.
Factors Influencing AF Performance
A camera’s AF system’s flexibility and capabilities are directly related to the number, position and type of the sensor points in its AF array. The more points the array has, the wider the area of the frame it can cover.
Most arrays contain sensors sensitive to different orientations; some vertical, others horizontal and a few ‘cross-type points’ sensitive to both directions. The more cross-type points there are in the array, the better it can provide precise focusing and the greater its ability to handle tricky situations.
The centre point in the array is usually a cross-type point and it’s almost always the most accurate point in the array. Most cameras can also use multiple AF points together to improve reliability or enable focus tracking. Some cameras include an ‘auto depth-of-field’ setting that groups AF points to keep subjects within the area at an acceptable level of sharpness.
Professional cameras generally have much more complex AF point arrays than consumer cameras, with a relatively large number of cross-type points. Some cameras allow photographers to configure the AF point array to suit different types of subjects.
The AF array of a high-end professional DSLR, showing the different orientations of the sensor points and their maximum aperture sensitivities. (Source: Canon.)
The number and accuracy of the usable AF points can change depending on the maximum aperture of the lens on the camera. Even if you don’t plan to use the lens at its maximum aperture, having a camera that can cover it should ensure the AF system is as accurate as possible.
One-shot (or single-servo) AF is the best option for static and slow-moving subjects. Continuous (or AI Servo) AF continuous re-samples the subject distance to predict where it will be and focuses on this point, taking account of possible shutter lag. This system is great for moving subjects but consumes some battery power.
Focus tracking involves more sophisticated algorithms that can allow for changes in subject velocity and direction. It is even more power-hungry than the continuous AF mode.
AF Configuration
Cameras designed for professional and enthusiast photographers often provide a range of AF system configurations. The most common is AF micro-adjustment, which tweaks the AF system to improve its accuracy with a specific lens.
Photographers can customise the built-in lens profiles with a calibration tool like Datacolor’s SpyderLensCal. It consists of a target, which is used to compare the camera’s focus at a pre-set distance with a calibration scale. Data obtained from calibration can be used to fine-tune the lens micro-adjustment settings in the camera.
Canon includes ‘profiles’ for its lenses in all cameras with this function. It has also developed a set of ‘Case’ profiles that configure the AF system for six commonly-encountered focusing situations, mainly based around different types of moving subjects. They are designed mainly for sports photographers and camera users can adjust tracking sensitivity, acceleration/deceleration tracking and AF point auto-switching to suit their individual requirements.
The Canon AF Microadjustment tool lets photographers match the profile of the lens to the camera’s AF system and tweak it, if necessary with a calibration tool. (Source: Canon.)
While these tools will certainly improve the accuracy and speed of a camera’s AF system, no camera can produce a perfectly focused image every time at every distance. And no adjustment procedure will change that. Good shooting technique, familiarity with your equipment, knowing how to set up your camera’s AF system for different subject types, and having a clear view through the viewfinder should stack the odds in your favour and make most of your shots sharp.
AF System Types
Phase detection is the main system used by DSLRs. A beam splitter, consisting of a small semi-transparent area on the DSLR’s main reflex mirror coupled to a small secondary mirror divides light passing through the lens. Two micro-lenses capture light rays from either side of the lens and direct them to a sensor, where they are analysed to find similar peaks and troughs in the light intensity signal.
This information is used to calculate the approximate distance to the subject. Analysing the separation between these peaks and troughs shows the direction and amount by which the focus must be moved.
Phase detection systems usually track subject movement easily. But they require fast data streaming and powerful data processors. AF systems in pro DSLRs are usually designed to handle higher data streams than consumer DSLRs and are better at subject tracking.
Contrast detection autofocus uses all the light passing through the lens to pick up the intensity differences between adjacent pixels. The lens is then adjusted until the maximum contrast is detected, at which point the image is in focus.
Most systems can only operate at the video frame rate of the camera, which can be relatively slow. Because they don’t calculate the position of subjects, they can’t be used for focus tracking.
Panasonic and Olympus have improved the contrast-detect systems in their M4/3 cameras by increasing the imaging frame rates to at least 120 fps. They also use faster AF drive motors in their lenses and more sophisticated processing algorithms.
Hybrid AF systems combine contrast and phase detection technologies and should, in theory, deliver faster and more accurate autofocusing. An array of phase detection sensors is embedded in the surface of the camera’s image sensor, as shown in the diagram above.
These sensors are used to obtain a rough estimate of where the main subject is when the centre AF point is used. The regular contrast-detection AF is used to fine-tune focusing and also when large focusing adjustments are required, such as moving from infinity to close-up subjects.
The arrangement of the optical components in a typical phase detection AF system. (Source: Canon.)
A typical hybrid AF array, showing the location of the phase-detection sensors embedded in the surface of the imager chip.
Viewfinder Dioptre Adjustment
If you can’t see a sharp image in your camera’s viewfinder, you can’t be sure the shot is focused. All cameras should have a dioptre adjustment for adjusting the sharpness of the AF points. It’s normally close to the viewfinder eyepiece.
Remove the lens from the camera and point it at an evenly-lit wall with a neutral beige or grey colour. Adjust the dioptre control until the AF sensor points in the viewfinder are as sharp and dark as possible.
If you can’t achieve a satisfactory adjustment, you may require corrective lenses, which are available through the camera manufacturer.
This is an excerpt from Photo Review Issue 55.
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