Focusing and depth of field

Depth of field in an image is greatest with small lens apertures, wide angles of view and distant subjects.

Depth of field varies depending on camera type, aperture and focusing distance and can also be influenced by the image size and viewing distance. It doesn’t change abruptly from sharp to unsharp, but instead occurs as a gradual transition.

Your choice of lens aperture controls how wide the area of acceptable sharpness is. Wide lens apertures will produce shallow depth of field, while small apertures can make everything appear sharp from close to the photographer out to the horizon.

When the subject occupies the same percentage of the image for both a telephoto and a wide angle lens, the depth of field in each picture will remain virtually constant. However, the zone of decreasing sharpness in front of and behind the focused distance will change with focal length, becoming less as the focal length becomes longer.

Depth of field is reduced in close-up shots, with narrow angles of view and when wide lens apertures are selected.

The table below shows how the depth of field changes as focal length increases for a subject at 10 metres from the camera with a lens aperture of f/2.8. This explains why it is critical to focus precisely with telephoto lenses.

How Cameras Autofocus

Most photographers use 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. Just about everyone relies on the latter because all cameras provide it. In DSLR cameras AF systems are generally fast, effective with most subjects and easy to use.

Two types of AF systems are common in DSLR cameras: phase-detection and contrast detection. Phase detection systems 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 usually 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 elements should be moved and in what direction, as shown in the diagram below. The lens motor will then move the optical components to provide correct focus.

Contrast-based AF systems 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 only used for the Live View mode in DSLRs, contrast-based AF systems are common on digicams and video camcorders.

They tend to be slower than phase-detection systems because they must drive the lens back and forth to find the position with greatest contrast. Phase-detection systems calculate the distance to the focus point and move the lens directly there.

The illustration above shows how the imaging light path is split in a DSLR camera to direct light to a beam-splitting sensor that measures how the lens elements should be moved to focus on the subject.

The diagram above shows the locations of the hybrid area (which contains phase-detection sensors)

As the focusing elements in the lens (simplified here) are moved, the signal reaching the line sensor varies and this is used to control the direction and degree to which the lens elements must be moved to achieve sharp focus. with respect to the full image area in a typical camera.

A typical AF point array in an entry-level DSLR.

The AF point array in a professional DSLR.

Both systems can fail when contrast is low. They can also 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.

Hybrid AF Systems

Hybrid autofocusing systems, as the name implies, combine phase-detection and contrast detection. They were first introduced by Fujifilm in a compact digicam, the FinePix F300EXR but other manufacturers, including Canon, Nikon and Sony have adopted the technology to provide faster focusing in the live view mode for their latest DSLR cameras.

The system requires an array of phase-detection sensors to be embedded in the surface of the camera’s sensor chip. (In Sony’s SLT-A99 there is already a phase detection module in the camera; adding phase detection sensors to the imager increases the speed and accuracy of the camera’s AF system in live view mode.)

How Many AF Points?

The camera’s AF area points are arranged in arrays within the image frame. Different cameras have different numbers of points and different array patterns.

Each AF array measures relative focus by evaluating local changes in contrast. The highest contrast is assumed to correspond to maximum sharpness.

The robustness and flexibility of autofocus is largely controlled by the number, position and type of points in each camera’s array. In general, the more AF points a camera has, the more sophisticated its AF system.

Two types of sensors are used in these arrays: linear and cross-type. Linear sensors can only detect contrast in one direction so, when they are used, some sensors may be orientated vertically, while others are horizontally aligned. This arrangement detects contrast in both directions.

Cross-type sensors have both horizontal and vertical detectors and can pick up contrast differences in both dimensions, making them more sensitive and more accurate. The more cross-type points there are, the faster and more accurate a camera’s AF system can be.

Multiple AF points can work together for improved reliability, or in isolation for improved precision, depending on the camera setting. The number and accuracy of the AF points selected in multi-AF modes can vary with the maximum aperture of the lens.

A central cross-type sensor can reduce focusing time for subjects in extreme defocus when using a lens with a maximum aperture of f/2.8 or brighter. When the maximum aperture of the lens is f/5.6 or smaller (a common situation for fast telephoto and long zoom lenses), surrounding AF points may be needed to focus the lens.

Spot autofocusing allows you to focus precisely on the area you wish to render with maximum sharpness.

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. In some cameras, groups of AF points can also be selected. Selected points light up in the viewfinder to show photographers which are in use.

One-shot AF is ideal for still subjects, like landscapes.

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.

In cameras with touch screens, touch AF/touch shutter controls makes selective focusing very easy. Touching a point on the screen causes the camera’s lens to focus on that area in the scene. When touch shutter is enabled, the shutter will be triggered once focus is achieved.

AF Modes

The most widely supported AF mode is one-shot focusing, which is best for still subjects. In this mode, 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 shot while maintaining the initial focus.

For moving subjects, most DSLRs include a continuous AF mode (also known as AF Servo). It adjusts the focus distance by predicting where the subject will be, based on estimates of its speed, and measured by continuously re-focusing. This setting improves your chances for getting sharp focus with moving subjects ““ provided they’re not moving too quickly ““ but constant re-focusing consumes more battery power than the single AF mode.

Tracking AF is a development of the continuous AF mode. It locks on to the subject when the photographer half-presses the shutter button 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.

Predictive AF is very effective for photographing subjects moving at a constant speed toward the camera. It can maintain accurate focus even when the subject is moving fast. If your target momentarily passes behind an obstruction, most cameras will hold and recapture the focus when the target emerges again.

Finding focus is more difficult when subjects are moving quickly across the photographer’s field of view. In such cases, cameras that offer Zone AF (which lets you select a group of AF points, either in the centre of the frame or off-centre) can make it easier to capture panning shots like the example shown.

This sequence of frames shows how the focus is passed from one point to the next in the AF point array in AF tracking mode as the subject’s position in the frame changes. (Source: Canon.)

In addition to Zone AF, most cameras provide users with a selection of AF area options. When the one-shot AF mode is selected, all sensor points are available and the camera will choose the ones that can focus on nearest subject with adequate detail.

In spot AF mode, a single sensor point is selected. The default is the centre point but most cameras allow users to select any one AF point and keep it on the subject.

AF Point Expansion, which is provided on more sophisticated cameras, adds several surrounding points providing a larger, moveable cluster of active AF points. This setting is especially useful if the central point in the cluster suddenly detects part of the subject with little detail or contrast.

The one-shot AF mode will usually focus quickly and accurately. Once you’ve acquired the target with the central sensor, the camera will pass it to other points automatically as the subject moves and, should it lose lock, it will attempt to guess where the subject is for about a second before starting to hunt for it.

Hunting drives the focusing elements back and forth within the lens barrel, searching for a high-contrast edge to lock onto. In poor lighting and when busy background or foreground items are included in the frame, the AF system can take several seconds to find the subject again.

Such delays can cause you to miss shots. When they occur, it’s time to turn to manual focusing.

An example of the use of predictive AF to keep the main subject sharp at a very slow shutter speed (1/15 second), while maintaining a good depth of field through use of an f/8 aperture setting.

To record this shot, the camera was panned in synchronism with the moving subject (the boat). The aperture and shutter speed were the same as the predictive AF shot (1/15 second at f/8) but the slow shutter speed caused background and foreground details to be blurred, isolating the subject.

Manual Focusing

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.

It’s important to start with a clear view of the subject. When using a viewfinder, make sure you adjust the optics to your eyesight with the dioptre adjustment.

Manual focusing is straightforward. Set the focus switch on the lens to M or engage manual focusing via the camera’s menu system. Select a focus point and frame the shot so the subject is behind it. Half-press the shutter button to initiate exposure metering then rotate the focusing ring to bring the subject into sharp focus before you press the shutter button all the way down to take the shot.

Many cameras allow you to use manual focusing to fine-tune autofocusing settings. This is handy when shooting close-ups with a hand-held camera and some lenses provide a manual over-ride in the AF mode for just this purpose.

Start by focusing automatically on something that isn’t moving but is at roughly the same distance from the camera as the subject you want to photograph. Then re-frame the shot on the subject and use your left hand to turn the focusing ring on the lens until the image is sharp.

Shooting with manual focusing takes practise ““ and the more you practise, the easier it will become. It pays to become proficient because you never know when it will be needed.

Tips for Minimising Blur

The best way to prevent blurring caused by camera shake is to mount the camera on a tripod ““ or set it up on a stationary object like a table or wall. When shooting moving subjects, you’ll need high shutter speeds to freeze action. At least 1/500 second is recommended for shooting sports action or wildlife or recording the actions of small children or pets.

Increasing the shutter speed reduces the amount of time light is allowed to reach the sensor. To compensate, open the lens aperture a stop or two, increase the ISO speed setting or use flash (or two or more of these at a time).

Opening the lens aperture lets in more light but also reduces the range of distances in the scene that appear sharp. Typically at an aperture of f/16, most of the scene will be in focus. At aperture f/4 or wider, only the subject the lens is focused on is sharp; the background and foreground will appear blurred.

Flashes are good for illuminating close subjects but their light is harsh. With an external flashgun you may be able to bounce the light off a light-coloured reflecting surface (wall or umbrella) instead of pointing the flash straight at the subject. Bounced light is softer and more natural looking but it takes practice to master the technique. Unfortunately, flash isn’t permitted in some places (theatres and sporting venues, for example).

Fast shutter speeds reduce the risk of blurred pictures when shooting subjects that could move erratically. They require you to open the lens aperture, which has the advantage of blurring busy backgrounds.

USEFUL URLs

/tips/shooting/lenses-and-focusing-basics. A basic guide to lenses and focusing systems.

/tips/shooting/focus-testing. How to test your camera’s AF system.

http://www.bythom.com/hyperfocal.htm. Advice about shooting for maximum sharpness.

http://www.bythom.com/autofocus2.htm. Using focus controls effectively.

 

This article is an excerpt from  Digital SLR Pocket Guide 3rd Edition.