Focusing options in a compact system camera

 Autofocusing Systems

Two primary autofocus systems are used in CSCs: contrast-based and phase detection. Both rely on contrast to achieve accurate focus. Some recent cameras offer a combination of both. Both systems rely on an array of sensor points arrayed across the sensor’s field of view to measure changes in contrast, although they operate in different ways.

This illustration shows the AF array in a typical entry-level camera. The central cross point can detect focus differences in both horizontal and vertical directions, while the outer points can only detect horizontal differences.

A more sophisticated grid-based array from a mid-level camera samples the scene according to different areas. Photographers can select individual areas or groups of areas, according to where they wish to focus and how much of the subject they want to focus upon.

Information from these points is compared to determine how much the focusing elements in the lens must be moved to make the image appear sharp. Although the calculations involved are complex and highly sophisticated, focus can be established within a fraction of a second, allowing the camera to lock on to the subject. Most cameras include the option to select individual points or clusters of points.

The speed and accuracy of AF systems depends on three factors: the number, position and types of focus detection points in the array; the direction(s) they are sensitive to; and the power and speed of the processing algorithms that make the focusing calculations. The type of motor that drives the lens also influences the speed of autofocusing as well as the noise created by moving the lens elements.

In the AF-Point-selection mode, the selected points are generally highlighted, as shown in the simulation above.

Contrast vs Phase Detection

Contrast-based AF systems measure differences in contrast at the sensor points. When high contrast is detected, the area is seen to be in focus. No actual distance measurement is involved, which means the camera can’t detect which way the lens elements should be moved.  

If the scene is grossly out-of-focus, it will move the lens, re-assess the contrast and, if it has increased, keep moving in the same direction until contrast decreases and it must move backwards. With low-contrast subjects and in dim lighting, the lens can rack back and forth, ‘hunting’ for focus. This can often take a second or more.  

Phase detection is based on the principle that when a point is in focus, the light rays coming from it will equally illuminate opposite sides of the lens (it is ‘in phase’). These systems are usually found in DSLRs but recently some CSC manufacturers have introduced ‘hybrid’ AF systems which have phase detection sensors embedded in the surface of the imaging sensor.

Phase detection systems can tell the lens AF motor which direction the focus is off (near or far) and how much to move to bring the image into focus. By enabling a constant comparison of distance information from different areas in the subject, they also support continuous autofocusing. When the array covers more than half of the image frame, predictive autofocusing and AF tracking are possible.

In a hybrid AF system, some pixel positions on the sensor can be used for measuring focus information, not image data. This doesn’t compromise the sensor’s imaging capabilities because fewer than 200 pixels are phase-difference detectors on a sensor with 16 million or more pixels.

The embedded phase-detection sensors aren’t as accurate as the more sophisticated stand-alone sensors used in DSLR cameras. But they perform well enough to provide rough directional information that speeds up the contrast autofocusing process.  

The latest cameras can focus almost as quickly as DSLR cameras for single shots and their continuous AF performance is improving rapidly, particularly for fast-moving subjects. Slower-moving subjects don’t present serious problems and continuous AF performance is similar to that of entry- to mid-level DSLRs.

In the Olympus Dual FAST AF system, 37 On-chip Phase Detection points (shown left) are used for tracking moving subjects, while 81 Contrast detection AF points provide high-precision autofocusing.  

Factors Affecting Focusing Performance

The three most important factors influencing autofocus are the light level, subject contrast and camera or subject motion. These factors aren’t independent. Most AF systems will find it difficult to focus in low light levels with low-contrast subjects. If either the camera or the subject is moving, that complicates matters.

Situations where autofocusing is difficult are best handled by choosing the most appropriate AF mode and then either selecting a focus point which corresponds to a sharp edge or distinct texture or over-riding the AF system and focusing manually. These decisions must often be made in a split second.

Low-contrast subjects in low light levels make autofocusing difficult.

AF Modes

All cameras provide at least two AF modes. One-shot focusing (Single AF) is the most widely supported and usually requires focus to be engaged by half-pressing the shutter button before the shot can be taken. The one shot mode can’t anticipate subject motion, making it ideal for stationary subjects.

With continuous AF, the focus distance is adjusted continually to keep moving subjects in focus. In most cameras, the AF system can predict where the subject will be and adjust the lens focus. Sophisticated systems are able to track both the speed and direction of motion of subjects.  

Actual tracking speeds depend on how erratically the subject is moving, the subject contrast and ambient lighting, the type of lens and the number of autofocus sensors used to track the subject. Unfortunately, focus tracking is very power hungry and can dramatically reduce the camera’s battery capacity.

Face Detection AF uses pattern recognition to help the camera’s AF system lock onto faces in scenes. Most cameras can identify eight or more faces automatically and they will prioritise faces closest to centre of frame. Some cameras include settings for pet recognition, thanks to algorithms that can detect cats’ and dogs’ faces.

Face detection AF systems can quickly identify and focus upon faces that are close to the edges of the image frame.

Detected faces are usually identified by superimposed coloured rectangles on the LCD or in the EVF. The primary face that will be used for focusing is shown in a different colour from the other faces. Face detection is particularly useful when shooting off-centre subjects, as shown in the illustration above.

Although no system is totally foolproof, the systems in today’s cameras are accurate more than 95% of the time. The only thing to remember is that outside of the fully automatic shooting mode, you may need to turn face detection on.

Tracking AF systems can keep moving subjects in sharp focus, even when the movement is towards the camera.

Some cameras let you ‘register’ the faces of family members and friends so they will be prioritised automatically in future shots. Some also include a face detection self-timer mode that lets the photographer press the shutter button and waits until they step into the picture, delaying the exposure by two more seconds before shooting.

Manual Focusing

There are times when manual focusing is a better choice or when autofocusing requires additional manual fine-tuning. Sometimes the latter is only accessible via the AF+MF setting in camera’s menu; at other times a switch on the camera’s body or the lens lets you swap between both settings. Situations where manual focusing is preferable include:

1. Some cameras and lenses have difficulty focusing in low light levels. This largely depends on the type of AF system they use.

2. Some subjects are better tackled with manual focusing when there are obstructions between the camera and subject (such as fences or glass). Manual focusing can also be better for shooting fast-moving subjects as you can pre focus on a point that the subject will move through and trip the shutter when that happens.

The very depth of field in close-up subjects requires precise focusing, which can be achieved by manual adjustment.

3. Manual focusing ““ or manual fine-tuning of autofocus settings ““ is recommended for close-up and macro photography because the narrow depth of field in these subjects requires precise focusing on the correct area in the subject.

Manual Focusing Aids

Modern CSCs provide plenty of aids to help users focus with precision in manual mode. The most common is focus magnification, which enlarges the area in the centre of the frame to let you see whether the image is sharp.  

The higher the magnification, the smaller the area covered by the enlarged display, and the easier it is to focus precisely. Most cameras provide between 5x and 10x magnification, with some going as high as 14x. Many also let you move the magnified area around the frame by using the arrow pad’s directional buttons. Cameras with touch screens may allow the enlarged area to be moved with a fingertip.

An increasing number of CSCs include ‘focus peaking’ displays (see Chapter 3), which outline areas that are in focus in a scene. The most common outline colours are white and black, although some cameras include red or yellow outlines. The peaking lines are not recorded in the photograph.

The third focusing aid emulates the split-focusing-screen in old film cameras. It is the least common aid but nonetheless effective and can be handy for subjects where contrast is relatively low.

Cameras with focus magnification let you zoom in on part of the frame when adjusting focus manually.  

The illustrations above simulate a split-screen focusing system. Aligning both pieces of the split area brings the subject into focus.

As indicated, the central area of the viewfinder screen is split horizontally. The lens is focused manually in the usual way and when both pieces are aligned the section covered by the split screen will be in focus.  

Focusing Motors

Autofocusing is driven by motors that control the speed at which a lens can be adjusted and how much noise the adjustment creates. Most high-performance lenses are driven by ultrasonic motors (USM), which are fast and very quiet. Stepping motors can also be comparatively quiet and efficient, particularly if they use piezo-electric technology.

Cheaper lenses are often driven by micro-motors, which can be noisy, particularly if they are geared. Some systems combine a micro motor with ultrasonic vibrations. They aren’t as fast as USM systems but are quieter than micro-motors alone.  

Focusing Tips

1. Still photos are best taken with the one-shot AF mode, using AF point selection to choose which parts of the subject are sharply focused. For portraits, the eye is the best focus point; for landscapes the centre of the scene, roughly a third of the distance between the camera and the horizon.

2. Continuous and Tracking AF are the best modes for action shots. Focusing speed and accuracy can be improved by ensuring the lens doesn’t need to search over a wide distance. Pre-focusing the camera at a distance you expect the moving subject to pass through will improve your chances of success. Be warned, shots can be taken when focus lock hasn’t been achieved in these modes.

 

Excerpt from Compact System Camera Guide.