Correct focusing is the key to a successful photograph. But how often have you looked at a shot and discovered that the areas that should have been sharp are slightly blurred? By understanding how your camera’s AF system works – and how to use it effectively – you can take sharply focused pictures more often and control which areas in the image are sharp.

 

Correct focusing is the key to a successful photograph. But how often have you looked at a shot and discovered that the areas that should have been sharp are slightly blurred? By understanding how your camera’s AF system works – and how to use it effectively – you can take sharply focused pictures more often and control which areas in the image are sharp.

We’ll start be investigating the situations that most commonly create focusing difficulties. These include low-contrast subjects, dim lighting, fast-movement, overlapping subject elements and factors that can interfere with a camera’s autofocusing performance, such as bright lights and glare. Each requires a special technique and, in each case, the photographer must decide on the most appropriate shooting mode for the subject.

1. Low-contrast subjects

Because the AF systems in most sophisticated cameras are contrast-based, the lack of contrast in the subject makes it difficult for the AF system to lock on to any object in the field of view. Subjects that are almost monochromatic can cause similar problems. The best solution is to switch to manual focusing and ‘guesstimate’ the distance to the most important element in the subject. For close subjects, use of a moderately small lens aperture will ensure the depth of field is wide enough to cover the area in which you want sharp focus. For more distant subjects, use of the infinity setting can also yield good results.

2. Overlapping objects

The main problem with overlapping objects is making the camera focus on the key subject in its field of view. This is especially true when the subject is off-centre. If your camera doesn’t support focus point selection (which lets you lock focus on a specific AF sensor), use the focus lock. Position the subject centrally in the field of view and half-press the shutter button. Then recompose the subject and press the shutter button the rest of the way down. An alternative solution is switching to manual focus for more precise control.

3. Low light levels

Contrast-based sensors perform poorly in dim lighting and, although an increasing number of cameras have AF assist lamps, these may not be powerful enough for distant subjects. Glare from bright lights can also confuse AF systems. Some cameras will not operate when they cannot focus. Switching to manual focus can solve many focusing problems. Increasing the ISO speed may also be helpful – but watch for an increase in image noise. Mounting the camera on a tripod will reduce the risk of camera shake.

4. Fast action

Photographers have two options when shooting fast action: using a slow shutter speed and panning, or using a fast shutter speed with the aim of freezing the action. Each has different constraints. Successful panning depends on the photographer’s ability to track the subject as it passes and press the shutter to ensure the subject is sharp while the background is blurred due to the camera’s movement.

Freezing action requires adequate light so you can set very fast shutter speeds (1/500 second or faster). Where ambient lighting is adequate, a high ISO setting can be useful. It’s easier to focus on subjects that are moving towards you than on those moving across your field of view. Use of a wide aperture can help to isolate the subject from an intrusive background.

For subjects within about four metres of the camera, light can be provided by a flash. An external flash is recommended for more distant subjects as it provides more light than built-in flash units, but few can reach beyond about 20 metres and close subjects are likely to be overexposed.

In most cases the shot will be exposed by the light from the flash and, as flash bursts are typically microseconds long, the shot will be clear and sharp. However, if your camera’s flash sync speed is slow (1/60 to 1/125 second) and ambient light levels are relatively high you may see some double imaging as both ambient and flash exposures are recorded in the shot.

Focus Mode Selection

If your camera has a selection of AF modes, use them to help you focus selectively. Multi-pointAF systems are best suited to subjects like landscapes, where you want most elements to look sharp. In many cameras you can select a specific focus point and focus on it. Spot AF systems work in a similar fashion, but usually with the focus in the centre of the field of view.

Some cameras include a continuous or ‘tracking’ AF setting, which constantly re-adjusts the focus as the subject changes position in the central field of view. This mode is great for tracking action but tends to consume a lot of power, so use it sparingly!

Use manual focusing for subjects in tricky lighting and in any situations where the camera’s AF system performs poorly (such as those listed in the main feature). Some cameras have infinity and macro settings, the first being useful for landscape shots and the second for close-ups. Make sure you know the close-focusing limit of the camera, when taking ‘macro’ shots.