Shooting

A stand-out factor that makes the difference between an ordinary snapshot and a successful photo you can frame and display is the position of the sharpest focus. This is the point that draws the viewer's attention so being able to decide where it will be in a picture is a very useful skill.

Photographers who use digicams have less control over the area of sharp focus than DSLR users but they benefit because the small sensors in digicams ensure most of the pictures they take are sharp. Camera manufacturers have also produced some handy face detection tools that help snapshooters to focus of human faces and ensure they are correctly exposed.

Camera Focusing Systems
Most cameras have an array of autofocusing (AF) points on a dedicated sensor. These are used to determine where the lens is focused. The exposure will also be metered at these points and shutter speed and aperture will be set in the full auto and P shooting modes.

A typical AF sensor pattern. The sensor in the middle is a cross-type sensor while the others are linear, although vertical and horizontal directions are included. The rectangles mark the approximate positions of the sensors in the viewfinder. The sensors, which are invisible to the eye, are shown in red (which is often the colour used to indicate which sensors are active).

All digital cameras use one or both of two systems:

1. Active systems fire a beam of infrared light at the subject and use the time taken for a reflection to return to calculate the subject distance. Common in simpler imaging devices like entry-level digicams and camera-phones, most active AF systems can't focus through windows, wire mesh or bars because the infrared light is reflected by the glass or wire. They may also fail with very close subjects because there's not enough time for the reflection to return to provide accurate distance measurements.

 2. Passive systems use the light that is naturally reflected by the subject and adjust the lens to deliver the maximum contrast within the sensor field. Two methods are used: phase difference and contrast-detect. Phasedetection separates the incoming light into pairs of images and compares them via a dedicated sensor, adjusting the focus of the lens until the images coincide. Contrast-detect systems measure the intensity differences between adjacent pixels in the image produced by the lens and adjust the lens until the greatest differences are obtained.

DSLR cameras with Live View may or may not support autofocusing in Live View mode. Those that do normally use contrast-detect  AF, which is noticeably slower than the normal phase-difference AF system. Pressing the shutter button flips up the mirror, briefly blacking out the viewfinder while the shutter opens and exposes the image sensor to the light from the lens. Autofocusing is slowed as a consequence.




The diagram left shows how a contrast-detect autofocusing system works, with a black rectangle representing the subject and the AF sensors shown as red rectangles. The sensor labelled A is the only one capable of detecting contrast because it spans a contrast boundary. Sensor B would see all white; sensor C would see all black and sensor D would see all grey (an equal mixture of black and white).

Both types of passive systems may be unable to find focus when the overall subject contrast is low and/or the AF sensor is unable to detect edges in the subject. Examples of such subjects include misty scenes and large single coloured surfaces like walls or blue sky. When this happens, the camera may ‘hunt' for focus, driving the lens back and forth through its focusing range.

Scenes in which the overall subject contrast is low will present problems for most autofocusing systems. To take this photograph, the AF point in the centre of the viewfinder was positioned over the boat in the foreground and the shutter button was half-pressed. The shot was then recomposed before pressing the shutter button all the way down to take the photograph.

Quick Tip: To identify your camera's AF system, point the lens at a clear area of blue sky (No clouds or other objects in the field of view). Halfpress the shutter button. If the camera focuses, it's an active system or a hybrid active/passive system; if the camera can't focus, it's a passive system.

Although point-and-shoot digicams and camera-phones seldom support manual focusing, most have a form of zone focusing that sets the lens to focus at a particular distance. Normally two ‘zones' are provided: macro (for close-ups) and infinity (for landscapes).

More sophisticated digicams and all DSLRs provide manual focus modes that require photographers to switch off autofocusing, either via the camera's menu or with a slider on the lens. Switching to manual focusing on a DSLR (and a few Advanced digicams) allows the lens ring to be turned to focus the subject. Some cameras maintainthe electronic link between the lens and the camera (a functioning battery is required to allow the focus ring to be turned and provide the focus confirmation light).

Quick Tip: When focusing manually, always check your shots to see the focus is set on the right part of the subject. Most cameras with manual focusing allow you to enlarge the shot in playback mode so you can confirm the critical zone in the subject is really sharp. 

AF Modes and Patterns
The AF systems in modern cameras are generally fast, effective with most subjects and easy to use. Most cameras offer several AF modes:
• single AF;
• continuous AF;
• predictive or tracking AF;
• multi-area focusing and spot AF.

Single-area AF is the most frequently-used focus mode. Pressing the shutter button half-way down activates autofocusing to achieve sharp focus on the subject in the centre of the screen. Only one distance measurement is made. The point of focus may flash briefly or be outlined with a bright square. In DSLR cameras, a focus confirmation light is displayed in the camera's viewfinder.

In continuous AF mode, the camera focuses continuously as the position of the subject changes. This is handy for shooting moving subjects - provided they're not moving too quickly. However, constant re-focusing consumes more battery power than the single AF mode.

The diagram shows the sequence of events that takes place with regular and tracking AF systems.

Predictive AF can track subjects moving towards or away from the camera, whereas tracking AF does better with subjects moving across the field of view.

Both systems work better with subjects that move at a constant speed than subjects whose speed and direction change rapidly. A signal - in the form of a confirmation light or beep - indicates when focus is achieved. More sophisticated cameras with predictive and tracking AF allow photographers to select 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 more than 50.

Spot focusing focuses the camera on a small spot in the centre of the viewing screen. It is used when precise focusing is required.

Face Detection
Face detection technology is now common in both digicams and entry-level DSLR cameras. When you switch on a camera with face detection activated, the autofocusing system - and usually also the auto exposure system - links with a microprocessor that analyses the scene, looking for areas that are skin coloured and shaped like human faces with identifiable eyes and mouth in the correct places. Animals' faces aren't detected so it's pointless using face detection for shots of pets and animals at the zoo.

In the early days of the technology, the algorithms used for the analysis could only detect faces looking directly at the camera. Early systems also had difficulty detecting faces that were upside down. The latest algorithms are multi-directional and can detect faces in full or semi-profile. Today's more sophisticated processing algorithms can also detect more faces in a scene, with some cameras capable of identifying ten or more faces.


Cameras with face detection technology will usually indicate the face used as the focusing target with a different coloured box from other faces detected in the scene. Faces that don't show the standard pattern of two eyes and a mouth may not be identified.

In most cases, the face detection system will confirm detection by overlaying a rectangle on each face in the scene displayed on the camera's LCD. Some systems use a different colour to identify the face that will be used as the focusing target. All systems analyse the brightness levels of the faces detected and set the exposure level for the shot on the basis of this information, favouring the face used as the focusing target.

Many systems link with the flash exposure system to ensure a natural-looking balance of flash and ambient lighting. Exposure adjustment to correct backlighting is also provided by a few recent systems. Tracking facilities are also provided by some recent systems. They can follow one or more faces when they move within the frame and automatically adjust focus and exposure when the camera's shutter button is half-pressed.

Cameras with face detection make it easier to ensure human subjects appear sharp and correctly exposed in photographs. They're great for novice photographers because they can handle off-centre subjects and shots with people on either side of the frame, whereas cameras without face detection will focus on the centre of the scene, leaving the main subjects unsharp. They can also improve your chances of taking sharp pictures of active children.

However, most systems will not identify faces that are too close to the camera - or too far away. If the camera ‘sees' only one eye and part of the subject's nose, detection could fail. Sunglasses, swimming goggles and other things that cover the eyes can also cause face detection systems to fail.

Scene Recognition and Postcapture Processing
Scene recognition is becoming increasingly common in both digicams and DSLRs. It works by analysing colour, brightness and distance information from the camera's sensors and comparing the resulting patterns with patterns stored within the camera that characterise certain subject types.

Most cameras with scene recognition technology can automatically identify which of the following categories a subject falls into: landscape, portrait, backlit portrait, twilight, night portrait and close-up. Once the subject type has been identified, the camera's aperture and shutter speed will be set, the focusing, exposure and sensitivity ranges may be constrained and colour adjustments may be made to deliver enhanced results for the specific subject type.

Smile Shutter and Blink Detection technologies are other recent technologies for providing snapshooters with more printable photographs. The former involves continuously scanning the subject and only triggering the shutter when a smiling subject is detected. (With some systems a broad grin is required.) Blink detection causes the camera to take several shots of the subject in a burst. Each shot is analysed and the camera saves the one in which most people's eyes are open.


Smile Shutter technology will normally only trigger the camera's shutter when a broad, cheesy grin is detected. It can be frustrating to take pictures in this mode because you don't know exactly when the shutter will fire.

Smile Shutter can be frustrating to use because the camera is effectively locked until a wide enough smile is detected. Blink Detection isn't foolproof in most cameras because it can't guarantee a good shot - only that you'll get a shot in which most people's eyes are open. Fortunately, both systems can be turned off.

In-camera red-eye detection is common in digicams. It works by analysing images after the shot is taken and looking for the characteristic red eyes that result from many shots taken with flash. The red patches are replaced by dark blue-black, making subjects' eyes look more natural (and less demonic). Some cameras can carry out the process on-the-fly, while others require you to select red-eye correction in playback mode. Both systems are equally effective - and neither is totally failproof.

Depth of Field
Controlling the width of the zone of sharp focus in a picture is one of the marks of a competent photographer. Experienced photographers refer to this zone as its depth of field. They use different camera settings to determine whether the zone is wide for landscape pictures or narrow for portraits and close-ups where they want to isolate the subject from a potentially distracting background.

An example of a photograph with a shallow depth of field. The focus is on the cat's nose, leaving the eyes slightly out-of-focus. Nikon D700, 60mm lens, 1/40 second at f/6.3.

Depth of field is controlled by three factors: the physical size of the sensor, the distance between the camera and the subject and the relationship between the lens and the lens aperture. The smaller the sensor the greater its inherent depth of field, which is why controlling depth of field is more difficult with digicams than DSLRs.


Creative use of a shallow depth of field to isolate a subject from distracting foreground and background elements. Nikon D3, 400mm lens, 1/50 second at f/5.

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 actual depth-of-field.

DSLR users can use the Aperture-priority (A) shooting mode to adjust the lens aperture for depth-of-field control. Wide apertures (typically from f/2 to f/4) will blur background details, even in wide-angle shots with a DSLR. In contrast, at f/16, depth-of-field is extended to a metre or more, with even a moderate telephoto lens (70mm to 100mm equivalent in 35mm format).

The skill is to stop the lens down just enough to make the background fuzzy without causing the key elements of the subject to be out of focus. Some cameras let you 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 below).


The arrow indicates the location of the depth-of-field preview button.

Suppose you want to make as much as possible look sharp in a scenic shot? You can achieve this with a DSLR camera by focusing on a point known as the hyperfocal distance. 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.

Using a long telephoto lens can help to narrow the depth-of-field in shots taken with a smallsensor digicam. The greater the zoom range, the shallower the plane of focus you can achieve and the easier it is to isolate the subject by blurring the background.

USEFUL URLs
The following websites provide additional information on the topics covered in this chapter.
www.photoreview.com.au/guides/digitalslr/focusingfocusing-and-depth-of-field.aspx for information on focusing and depth of field.
http://en.wikipedia.org/wiki/Depth_of_field  for a comprehensive guide to depth of field and related topics.


This is an excerpt from Mastering Digital Photography Pocket Guide 2nd Edition.
Click here for more details on this and other titles in the Pocket Guide series.


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