Digital SLR

The focus control is used to make subjects look sharp in photographs. All DSLR cameras come with both manual and automatic focusing (AF) controls. The AF systems in modern DSLRs are generally fast, effective with most subjects and easy to use. Essentially there are two types of focusing, manual focusing in which the photographer focuses on the subject by turning the focusing ring on the lens and autofocusing (AF), where the camera focuses on the subject.

Manual focusing is straightforward. Provided the subject is clearly seen in the viewfinder, it should be easy to see when it is in focus and you can nudge the focusing ring slightly to correct any residual unsharpness. Focusing manually can often be quicker than autofocusing, particularly when subjects are moving rapidly either towards or away from the camera. It may also be more precise with close subjects and it allows the photographer to decide which part of the subject has the maximum sharpness.

Two types of autofocusing systems are common in digital cameras: active and passive. Active AF systems use an infrared or ultrasonic beam to measure the distance from the camera to the subject and adjust the optical system accordingly. This type of AF is common on video camcorders and some compact digicams. It is seldom found on DSLRs.

Passive AF systems analyse the light entering the camera through the lens (TTL) and set the focus accordingly. Two types are in common use, phase-detection and contrast measurement. Phase detection systems, which are generally favoured for DLSR cameras, work by dividing the incoming light into pairs of images and comparing them. The system uses two optical prisms which capture the light beams passing through opposite sides of the lens.

 

A beam-splitting, semi-transparent mirror (or a semi-transparent area on the main reflex mirror) directs these beams down to an AF sensor, which is often located below the mirror. The two images are then analysed to find similar waveforms. The phase difference between the two images is used to determine how much the lens should be moved and in what direction. The lens motor will then move the optical components to provide correct focus.

Contrast-based AF systems, which are commonly used for the Live View mode in cameras that support this function, rely on measuring the intensity difference between adjacent pixels on the AF sensor. This intensity difference reaches a maximum when the lens is correctly focused. Although seldom used on DSLRs, contrast-based AF systems are common on digicams and video camcorders because they are fast and technologically simpler to implement.

Each type of AF system has specific weaknesses that photographers should be aware of. Active systems can't focus through window glass, mesh or bars because these impediments will reflect the IR beams.  They may also fail with very close subjects. However, because they emit a beam of radiation, active systems can usually focus in total darkness, when required.

Passive systems can fail when contrast is low because they rely on subject contrast. They are most likely to fail in dim lighting or when the subject contains large areas of a single colour (sky, wall, etc.). Many cameras include an AF illuminator to overcome such problems, although it will only work with relatively close subjects.



Passive autofocusing systems can fail with low-contrast subjects, such as misty scenes. (36mm focal length and ISO 400 sensitivity, 1/60 second at f/7.1.)

AF Sensors
All DSLR cameras use an array of AF area sensors to detect the signals that are used by the AF systems. Different manufacturers use different sensor patterns and many of them use a combination of rectangular and cross-type sensors for greater accuracy. Rectangular sensors provide two-dimensional intensity patterns, while cross-type sensors have a pair of rectangular sensors orientated at 90 degrees to each other.


A typical AF array with nine cross-type sensors.

Cross-type sensors can recognise both vertical and horizontal movement patterns and, therefore, increase the accuracy of the AF system. Modern cameras can also analyse the subject and select the most appropriate AF area to focus on, further increasing focusing accuracy. Some sensors include a mode that will combine adjacent sensors in groups to improve focusing speed without compromising accuracy.

Some cameras allow you to select a single AF point for focusing and will normally highlight the selected area in the viewfinder. Others allow users to focus with a group of sensors. A few provide a special 'closest subject' mode that will prioritise the closest subject detected by the AF sensor array.

Single and Continuous AF
Most DSLRs offer both single (S) and continuous (C) AF modes. In single-area AF, the most frequently-used focus mode, the default setting focuses the camera in the centre of the screen. Pressing the shutter release half-way down activates the autofocus and achieves sharp focus only once. The focus is retained while the shutter button is held down, allowing the photographer to re-compose the shots while maintaining the initial focus.

In cameras with multi-area AF sensors, the AF points that achieve focus flash briefly and a focus confirmation light is displayed in the camera's viewfinder. (The exposure will also be metered at the focus points and shutter speed and aperture will be set in the full auto and P shooting modes.) The single AF mode will usually provide the best results for sports shots and situations where you wish to select a subject from a group


Single-servo autofocusing is best used for sports shots when you want to select a subject from a confusing background. (800mm lens, 1/125 second at f/16. Image supplied by Canon.)

In the continuous AF mode (also known as AF servo), the camera re-focuses on the subject continuously, regardless of the position of the shutter button. This mode is handy when shooting moving subjects - provided they're not moving too quickly - but constant re-focusing consumes more battery power than the single AF mode.


Continuous AF is effective with moving subjects because the lens is re-focused continuously. (100mm lens, 1/125 second at f/6.4.)

If there is a brief time lag between when the shutter is pressed and when the picture is recorded, moving subjects may not be pin-sharp in this mode. Predictive and tracking AF systems have been developed to solve this problem.

Predictive Focus Tracking
Many DSLR cameras include predictive focus tracking, which is useful for photographing subjects that are moving at a constant speed. When the photographer half-presses the shutter button, the AF system locks on to the subject and determines its speed and direction of motion.

A servo feedback mechanism allows the system to predict where the subject will be when the shutter button is pressed all the way down. A signal - in the form of a confirmation light or beep - indicates when focus is achieved. Note: for subjects to be sharp you need to track the subject for a second or two before taking the shot. This system won't work for subjects that are moving rapidly but erratically, such as children on swings, pets in motion and some types of sports.

AF Point Selection
Most DSLR cameras allow photographers to choose which one of an array of focus points the camera will use for focus and exposure determination. The number of points varies, with some cameras offering three selectable points, others five and yet others nine - or more. The selected point lights up in the viewfinder to show photographers which AF sensor is in use. In some cameras, groups of AF points can also be selected.
Another way to select a limited range of AF sensors is to use the Spot AF mode. This focuses the camera on a small spot in the centre of the viewing screen. It is used when precise focusing is required. Unlike the AF point selection setting, this mode always focuses in the centre of the frame.

Selective Focusing
Controlling the width of the zone of sharp focus in a picture is one of the marks of a competent photographer. This zone can be wide or narrow, depending on the depth of field used for taking the shot. Depth of field is defined as the zone of acceptable sharpness in a photograph - or the distance in front of and behind the subject which appears to be sharp.


Selective focusing allows you to blur out distracting backgrounds. (55mm lens, 1/50 second at f/2.8. Image supplied by Canon.)

The size of this zone is controlled by three main factors: the lens aperture, the distance between the camera and the subject and the relationship between the lens and the size of the imaging area. In practice, it can also be influenced by how much the photograph will be magnified in the printing or viewing process and from what distance it will be viewed. These factors should also be considered when taking pictures.


A large lens aperture (in this case f/2.8) narrows the depth of field in the image, causing the background to be blurred.


A small lens aperture (in this case f/22) widens the depth of foeld, allowing background details to be resolved.
 
Large (wide) apertures produce very shallow depth of field, while small apertures make everything sharp from close to the camera to the horizon. The illustrations above show just how important the lens aperture can be in determining what is and isn't sharp in a picture - and, thus, where the viewer's eye will be directed.

Most photographers try to reduce depth of field when taking portrait shots to isolate the subject. The skill is to stop the lens down just enough to make the background fuzzy without causing the key elements of the subject's face to be out of focus.

Start by setting the lens to its widest aperture - or one stop down if you're shooting with a focal length of 80mm or longer. If the background still looks too sharp, try moving the subject away from the background and move back so you can use a longer focal length lens. You can see how much depth of field you have in a shot before you take it by pressing the depth of field preview button on a DSLR camera. It's usually positioned low on the camera body next to the lens mount (see illustration below).


The arrow indicates the location of the depth-of-field preview button on the EOS 40D camera.

Tele lenses are easier to use for selective focusing than normal or wide angle lenses. (The longer the lens focal length, the shallower the plane of focus you can achieve.) It's also easier to achieve shallow depth of field with 35mm film SLR cameras or DSLRs with 36 x 24mm ('full frame') sensors than DSLRs with smaller ('APS-C sized') sensors. With most compact digicams selective focusing for restricted depth of field is almost impossible due to their small sensor sizes, except in macro mode.

Wide angle lenses appear to have more depth of field than telephoto lenses because they include more of the area surrounding the subject. However, when the size of the subject in the frame is identical and the other key parameters (lens aperture and sensor size) are the same, lens focal length on its own has no effect on depth of field.

Suppose you want to make as much as possible look sharp in a scenic shot? Maximum depth of field is obtained by focusing on a point known as the hyperfocal distance. If you focus on this point, everything should be sharp from half way between it and the camera right out to infinity. A quick way to set the camera to the hyperfocal distance is to focus on infinity then re-focus on the nearest point that appeared sharp when the lens was focused on infinity. If you set the lens aperture to f/11 or smaller, the subject should appear sharp from one third of the distance between the object the lens is focused on and the camera to two thirds of the distance behind the focus point.

USEFUL URLs
The following websites provide additional information on the topics covered in this article.
www.photoreview.com.au/guides/digitalslr/focusing-focusing-and-depth-of-field.aspx for information on focusing and depth of field.
en.wikipedia.org/wiki/Depth_of_field for a comprehensive guide to depth of field and related topics.
web.canon.jp/imaging/enjoydslr/index.html for a general guide to using a DSLR camera.