Some cameras never need to be focussed. That’s the way disposable cameras and phone cameras, and many underwater cameras work. They have a “fixed focus lens” with a very small aperture, which gives them a very large depth of field. Unfortunately, that means they can’t be used in low light without a very fast film or a super sensitive sensor. It also means that you cannot use one of your camera’s prime creative tools: selective focus Some medium format cameras and all large format cameras use body based focusing. The body of the camera itself, or a set of tracks attaching it to the lens, is moved. These cameras, along with tilt-shift lenses for other cameras, often allow for selective focus of different objects at the same distance. They are mentioned here mainly to emphasise that what we take for granted as ‘best’ or ‘most modern’ may not impress those people with the biggest investment in top end gear.
There are also still a few digital rangefinder cameras about. They are not common and confined to a few expensive niche brands. Rangefinder cameras use a split image in the viewfinder produced by an offset viewing window and a movable mirror. The control for the mirror is attached to the lens focusing mechanism. Focusing involves lining up the two images in the viewfinder. When they are aligned, the object in focus. Rangefinders and other split-image focusing aids were once the standard focussing method.
Automatic Focus (autofocus) is the standard on modern cameras and for snapshot photographers who simply want to capture a memory, autofocus is good enough. Autofocus is not always the best option, however, which is why all single lens reflex, and many of the better-specified compact cameras, allow you to over-ride autofocus.
Selective focus is a technique used to draw the viewer’s eye to the part of the picture you want it to go to, by making that aspect of the picture sharp while leaving other parts out of focus and therefore blurred and hard to distinguish. Obviously that isn’t possible if you can’t control focus.
Manual focus is still very useful. Autofocus is not perfect, and a lot cameras will not operate when they cannot focus; nor do all lenses have autofocus. Some of these manual focus lenses are the high quality choice of advanced photographers who prefer manual focus to get the photograph they have in mind. Many digital SLR cameras rely on the the autofocus motor to be built into the lens. Using older lenses requires focusing manually.
Autofocus doesn’t always cope - A number of situations can defeat autofocus systems. These include low-contrast subjects, dim lighting, fast-moving subjects and pictures where important elements overlap each other. Glare and transparency also make autofocus less reliable. Various approaches can help, and using different manual and autofocus techniques is the key.
Low Contrast - Autofocus systems rely on contrast in the area of the scene on which their sensors are trained to find focus. Where there is little contrast they struggle, “hunt” for focus, and often fail. A grey sky, large areas of similar tone or any area in low light are examples. Subjects that are almost monochromatic can cause similar problems. The simple solution is to switch to manual focusing. With an SLR your eye may well be able to judge when the subject is sharp even though the camera couldn’t. If not, or if your camera hasn’t got a through the lens viewfinder, estimate the distance to the important elements or focus on an object at a similar distance, then recompose and shoot. For distant subjects, use of the infinity setting can also yield good results. If your camera does not have an “infinity” setting, use the Landscape mode, which in effect, turns it into a fixed focus camera.
Overlapping objects - When objects overlap the camera has no way of knowing which is to be in focus. Trying to focus on a bird in the branches of a tree is a nightmare as the camera switches focus from on twig to the next, picks up a leaf, switches (too fleetingly) to the bird... Or trying to photograph a wrought iron fence when the camera wants to pick up the building behind it, or vice-versa. This is gets even worse if the subject is off-centre. If your camera support focus point selection, choosing the right sensor can help, but even so, focus lock is your best solution.
Position the viewfinder focus point over the subject and half-press the shutter button. Then recompose the subject and press the shutter button the rest of the way down. An even better alternative for our bird problem is to use either manual focus or exposure lock to focus on a point where the bird is likely to land (a little observation will often show a pattern of returning to the same branch, or the nest). Then wait for the bird to come into focus instead of trying to focus on the bird!
Low light and Glare - Contrast-based sensors don’t do so well in poor light, which is why cameras have autofocus assist lamps. Beyond about 2 metres (6 feet) they are not much help. Placing a bright or reflective target in the scene, locking focus on that then removing it before the exposure works well when practical.
The techniques under Low Contrast also help with low light. Consider using flash or adding more light ( for interior work, architectural photographers often replace standard light bulbs with high intensity photofloods). and switching to manual focus can solve many focusing problems. Increasing the ISO speed may sometimes be helpful - but watch for an increase in image noise. Mounting the camera on a tripod will reduce the risk of camera shake.
Glare and reflections also confuse autofocus systems. Blocking glare with a lens hood, your hand or hat (make sure you keep them out of the picture) solves many problems - flare as well as glare. If the problem is a reflection or glare on glass when you are trying to take a picture through a window, it might be enough to place the camera very close to the glass. If that isn’t enough, use the “infinity” technique )above).
Fast Movement - Whether you go with a slow shutter speed and pan with your subject, or choose a fast shutter speed to freeze the action, your autofocus system is still going to struggle.
Panning is easier, because the subject stays a constant distance from the camera as you track it, but “shutter lag”, the time between pressing the shutter button and the picture being taken, can easily spoil the picture. Half press the shutter release as you start to track; this will lock both exposure and focus on most cameras, but when you depress the button the rest of the way, there should be no lag - the camera will already have processed the exposure and other data and be ready to record your image.
Freezing action requires enough light for very fast shutter speeds, which might also require a higher ISO. It's easier to focus on subjects that are moving towards you than on those moving across your field of view because the relative change in the image on your sensor is much smaller.
You can, of course, set the camera to Sports Action mode. This tells the camera to do two things: select a high shutter speed and change the autofocus mode from single shot to continuous.
You can do this yourself in many cameras, and gain more control. Continuous is sometimes referred to as AI ; the difference between this and one-shot mode is that if you continue to hold down the shutter button halfway, the camera will continue to adjust focus as your subject moves around the viewfinder. The advantage: no shutter lag and a good chance of catching the action; but it also allows the shutter to fire whether the subject is in focus or not. This mode consumes a lot of battery power, so use it sparingly!
Some cameras also turn on burst mode when you select Sports/Action. It is worth considering, as it increases your chance of at least on well focussed and exposed shot from the set. However, if your camera can manage at least 3 frames per second, you are probably as well off staying with single shot.
Focus Mode Selection - In some cameras you can let the camera decide which of its 3, 5, 9. 11 or even more of its focus points is the one you want to focus on. You can also over-ride and choose for yourself. Obviously, it isn't always going to get it right. Other cameras offer a choice of single point or wide-focus while others always focus on whatever is in the centre of the viewfinder. It is important to know where your camera focuses if you are going to get the best from the system. This also applies to the recent slew of cameras with "face recognition" ability. It sounds useful, but can lead you to let the camera make your decisions!
Autofocus works very well in most typical photographic scenes; But not perfectly. Technique, lighting, subject matter and other factors influence autofocus.
