Techniques for obtaining sharp images in most situations.
Sharp images contain detail and clarity in the most important areas of the frame. In portraits of living creatures, the subject’s eyes must be sharp.
We’ll begin this feature by defining what constitutes a ‘sharp’ image and the main features that contribute to it. Ideally, sharp images contain as much detail and clarity as the camera and lens can record in the most critical areas of the image ““ which are determined by the photographer.
Sharpness is influenced by two elements: focus, and the stability of the camera while the exposure was recorded. When both are optimised, images will be sharp. Failure of one or the other results in blurring that can’t be corrected by most image editors. (Photoshop CC is the exception, with a new Camera Shake Reduction filter that can ‘rescue’ motion-blurred photos.)
Camera Focusing Systems
Most cameras offer 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. Check out the Insider feature on pages 36 and 37 of this issue for details of the latest autofocusing systems.
The most widely-used AF mode is one-shot focusing. 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.
Continuous AF (also known as AF Servo) continuously re-adjusts the focus distance by predicting where the subject will be, based on estimates of its speed. It’s designed primarily for use with moving subjects ““ provided they’re not moving too quickly. However, 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 shutter button is half-pressed and determines its speed and the 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. Most cameras provide a confirmation light or beep when focus is achieved. With most systems, you need to track the subject for a second or two before taking the shot.
Predictive AF is a development of tracking AF, designed for photographing subjects moving at a constant speed toward the camera. It can maintain accurate focus even when the subject is moving fast. If the target momentarily passes behind an obstruction, most cameras will recapture the focus when the target emerges again.
Focusing becomes more difficult when subjects are moving quickly across the photographer’s field of view. Cameras that let you select a group of AF points, either in the centre of the frame or off-centre, can make it easier to capture sharp images in such situations.
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 adds several surrounding points to provide a larger, moveable cluster of active AF points . It’s handy if the central point in the group suddenly detects part of the subject with little detail or contrast.
Manual focusing is often quicker than autofocusing, particularly when subjects are moving rapidly, and many cameras and lenses provide manual over-ride in AF mode. Simply frame the shot, half-press the shutter button to meter the exposure, then rotate the focusing ring to bring the subject into sharp focus before you press the shutter button all the way down.
Although the latest cameras may boast highly sophisticated focusing systems, they aren’t totally fail-proof and, even though some have been designed to suit the widest possible range of shooting situations, you need to know how to set the focus ““ and some other camera parameters ““ to ensure subjects are recorded with sharpness where you want it. There are no ‘magic’ settings that guarantee perfect results in all (or even any) situations; the settings you choose will depend on the subject you’re shooting and the situation you’re in.
Camera Stability
The most common cause of blurred photos is camera shake, caused by shooting with the camera hand-held and using a shutter speed that isn’t fast enough to overcome the photographer’s unintentional unsteadiness. Each of us differs in how steadily we can hold a camera and, for most of us, it’s a skill that can be developed.
However, it’s more difficult to hand-hold longer lenses at relatively slow shutter speeds than wide-angle lenses because they magnify the image (and any shaking) more. For this reason, stabilisation is often built into longer lenses, while it’s rare in wide-angle ones.
One-shot autofocusing was used to capture this interesting silhouette of a penguin from below the surface of the water. The camera was pre-set to use a cluster of AF points near the lower right hand corner of the frame and the lens aperture was set at f/8 to ensure adequate depth of focus.
Effective stabilisation was essential for this shot of a Samango monkey, which was photographed in a fig tree in the late afternoon, using a 600mm lens with a shutter speed of 1/125 second at ISO 800. The camera’s stabiliser compensated for camera shake but could not prevent the motion blurring of the monkey’s paws.
Subjects moving quickly across the photographer’s field of view are difficult to focus upon, particularly in low light levels. Panning with the subject enabled this New Zealand falcon to be ‘stopped’ in its flight with a 300mm lens set at f/5.6 with ISO 200 sensitivity. The AF system was set to use the central array of AF points, using AF Point Expansion. The 1/90 second shutter speed captured the rapidly flapping wings as a blur.
Manual focusing is usually more effective when photographing small, fast-moving birds, like this White-bellied Sunbird. The technique is to set up the camera on a tripod, with the lens pointed at a place you expect the bird to visit, focus on the target area, adjust the lens aperture and wait for the bird to arrive within the frame.
Stabilisation is useful in dimly-lit situations but it’s not always necessary, particularly if you have steady hands. For this shot a high ISO setting (ISO 3200), combined with an 85mm lens at f/8 (to provide good depth of field) enabled a shutter speed of 1/80 second to be used, which is just fast enough to hand-hold the camera without stabilisation.
There’s a useful rule of thumb for determining the slowest shutter speed you should use when shooting with an unstabilised lens (or camera). Known as ‘the reciprocal rule’, it states that when hand-holding a camera, the shutter speed should not be slower than the reciprocal of the effective focal length of the lens you are using. So, if you’re using a 200mm telephoto lens, the minimum shutter speed should be faster than 1/200 second.
Scenes with a lot of fine detail in the foreground need to be sharp from near the camera to the horizon. This is easier with a wide-angle lens (in this case, 28mm) but helped by the choice of an aperture of f/11.
Sharpness all the way to the end of this very long corridor was essential for the success of this picture. Use of the hyperfocal distance with a wide-angle lens and an aperture of f/8 enabled it to be achieved.
Misty scenes needn’t be sharp all the way to the horizon, particularly when it is obscured. The optimum focusing zone for this image is the cluster of reeds in the lower third of the frame. An aperture setting of f/11 ensured adequate sharpness as far as details could be discerned.
This rule works for people with fairly steady hands. If you have any doubts about your ability to hold a camera steady ““ or if you obtain slightly unsharp pictures when following this rule ““ add half to one stop, which brings the recommended shutter speed to 1/300 or 1/400 second.
Stabilisation
Image stabilisation, either in the camera body or the lens, can provide between two and four stops of shutter speed compensation, which means that instead of needing 1/200 second with a 200mm lens, you should be able to hand-hold it at 1/50 second (or maybe even slower). This enables you to use lower ISO settings or smaller lens apertures than the conditions would otherwise demand.
But stabilisation isn’t the be-all-and-end-all solution; it does not prevent motion blurring caused by subject motion or extreme camera shifts. For both, you need a shutter speed that will stop that movement.
Stabilisation also has limitations. The sampling frequency of the motion detection mechanism in the stabilisation system determines what kind of and how much movement can be removed. In most systems, the sampling frequency is around 1000Hz, which is equivalent to a shutter speed of 1/1000 second.
However, the camera’s image processor can only resolve data accurately below half the sampling frequency, which means it can only compensate for movements at shutter speeds below 1/500 second. So, if you’re shooting with higher shutter speeds, it’s best to turn the stabiliser off. If you don’t you may find the system is correcting out of synch with the shutter speed, which produces slightly blurred photos.
Most cameras only provide generalised stabilisation. More sophisticated models offer two stabilisation modes: one provides compensation for both vertical and horizontal movement, while the other compensates only for vertical motion. Use the former to counteract vibrations from shaky hands or when standing or sitting on a moving platform (car, boat, train, etc); use the latter when tracking moving subjects for panning shots.
In most cases, stabilisation should be switched off when the camera is tripod-mounted. Some cameras and lenses claim to be able to detect this situation and provide compensation. But, even in those cases, it’s usually best to turn stabilisation off.
Leaving stabilisation on when the camera is tripod-mounted can cause conflicts with the camera’s autofocusing system. In some situations, the stabilisation may detect autofocusing movements in the lens and attempt to correct them, creating conflicts between the two systems. Image sharpness is compromised by these conflicts, as we have determined with Imatest testing.
Where to Focus
One of the most vital skills of a good photographer is knowing where to focus and how much of the subject should be sharp. Focus goes hand-in-hand with depth of field; some subjects look best when a relatively shallow plane in the subject is in focus, while others require as much as possible of the scene to be sharp.
For living creatures, the optimal point of focus is the eye nearest to the camera. Although the central AF point is usually most sensitive, the place where you focus should be different. Switching to another point will deliver better results.
For scenery and architectural subjects, focusing at the hyperfocal distance delivers sharpness from half way back to the camera right out to infinity. You can establish the hyperfocal distance by focusing on the most distant object in the scene and then manually adjusting the focus until it’s as close as possible while the background object remains acceptably sharp.
If the scene doesn’t extend all the way to the horizon, or excludes the near foreground, focusing one third of the distance into the scene from the camera should achieve maximum sharpness throughout, provided the lens is stopped down sufficiently (between f/8 and f/11 usually provides the optimal aperture setting).
When using these calculations, take account of the elements in the subject. A finely detailed foreground may demand more sharpness than a hazy background. Alternatively, a naturally soft foreground can often sacrifice some sharpness for the background.
Taking Sharp Photos of Moving Subjects
1. Acquire focus first. Make sure the subject is in focus before pressing the shutter release. This appears self-evident but, when you’re tracking moving subjects, starting with the subject out-of-focus causes the tracking AF system to struggle, first to find focus and then to keep pace with the subject’s motion.
2. Allow for viewfinder blackout. If you’re using a DSLR camera, allow for the viewfinder blackout time. When the viewfinder is blocked by the mirror being up, the autofocus system is, too. It may only be for a fraction of a second but, if you’re recording bursts of shots and the AF system had no prior tracking information, it will struggle to keep pace and some shots will be unsharp.
3. Allow for available light. Set the ISO to Auto and use the menu system to determine the maximum light levels the camera can use, keeping in mind the amount of noise in high-ISO images. Don’t reduce the light reaching the sensor more than you have to by adding teleconverters and/or polarisers. For high-speed continuous shooting, turn off noise reduction processing to optimise capture rates.
4. Learn to anticipate movement. Most photographers’ normal practice is to use centre spot focusing and place the sensor where a moving subject’s head is likely to be. You’ll obtain a better composition if you check the direction in which the subject is moving and shift the AF point (or cluster) to that side of the frame. This enables you to fill the frame with the subject and have the subject’s eye in focus.
Tracking AF kept the eye of this leopard sharp as it moved steadily across the field of view. Use of off-centre AF sensors was required to fill the frame with the subject while keeping the eye sharply focused. An aperture setting of f/7.1 was wide enough to blur background and foreground grasses yet provide adequate detail in the subject. There was plenty of light so ISO 200 sensitivity was fine for the 200mm focal length of the lens.
This is an excerpt from Photo Review Issue 58.
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