Why do I need to take control of focusing when it's supposed to be automatic focus on my camera?
Like any other automatic feature of a camera, autofocus (AF) doesn't always get it right. The AF system can end up focusing on the wrong part of a scene, and there will be times when it struggles to lock onto anything, with the end result of a blurred picture.
Don't get us wrong: today's DSLRs and lenses can focus faster and more accurately than they ever could; but, as with metering and exposure, you need to be actively involved in the process to get the best results.
In what sort of conditions is autofocus likely to get it wrong?
Your camera can struggle to autofocus when there's not enough light or when it's faced with a low-contrast or uniformly coloured scene, such as a brown dog in a muddy field. There won't be any clear edge for the AF system to find.
In these situations, the lens will focus back and forth, hunting for something to lock onto. If there's anything in front of the subject, such as a window or the bars of a cage, chances are the camera will focus on that instead.
Moving subjects can present a significant challenge to an AF system too. You need to make that you've selected the right focus mode in order to stand a fighting chance of getting a sharp action shot.
So which focus modes should I be using, and when?
The first thing to decide is whether you want to use autofocus or switch to manual focus. There are some situations where manual focus is the better option, which we'll come onto later. Let's assume you're sticking with autofocus for now, in which case double-check the switch on the lens is set to AF and not MF.
Autofocus offers two distinct modes, which you need to set on the camera. These are One-Shot AF (Canon)/Single-Servo AF (Nikon), and AI Servo AF (Canon)/Continuous-Servo AF (Nikon).
The One-Shot/Single-Servo option is the best choice for stationary subjects. Once the autofocus system has achieved focus, it locks that setting in: you can take a picture, knowing that the subject will be sharply focused.
As the name suggests, AI Servo/Continuous-Servo focuses the lens continuously, making it a good choice for tracking a moving subject. By default, the camera will let you take a picture at any time, even if the subject isn't in focus.
Many cameras offer a third AF mode: AI Focus AF (Canon) or Auto AF (Nikon). This automatically detects whether the subject is stationary or moving, and switches the autofocus accordingly. For reliable results though, you should select the dedicated AF mode yourself.
Choosing the AF mode shouldn't be confused with choosing the AF Area mode, which can also be set automatically or manually.
SEE MORE: How to choose the right AF mode
What's the difference between AF mode and AF Area mode?
While the choice of AF mode dictates how the lens will be focused, the AF Area mode determines where the camera will focus. The way in which you access AF Area mode varies between camera models, but the options available are largely the same.
You can select either a single AF point or a cluster of AF points, or make all of the AF points active. Look through the viewfinder as you make adjustments, and you'll see the AF point configurations being highlighted as you go through the options.
How many AF points should I use?
It depends on what you're shooting. If you activate all the AF points, then the camera will automatically select which one(s) it will use to focus once you activate the AF system.
Although it's likely to pick out your chosen subject if it's large enough in the frame, you have no control over what the camera determines should be in focus. There's a chance that it could lock onto something in the foreground or background, or an incorrect part of the subject, such as the tip of a portrait-sitter's nose rather than their eyes.
However, this mode excels when you're photographing active subjects against a clean background, such as a bird flying across a blue sky. The more AF points your camera has, the more accurately it will be able to track the subject as it moves through the frame.
For the majority of situations, autofocus will be faster and more precise if you change the AF Area mode to Single AF Point instead.
Out of all the AF points available, the centre one offers the greatest precision; take advantage of this by pointing it at the feature you want to be sharp and half-pressing the shutter release to activate and then lock the focus.
Then, with the shutter release still half-pressed, recompose your picture and press the shutter release fully to take the shot.
When should I use manual focusing?
Manual focusing is for when you want the focus distance to remain constant. For example, by autofocusing on a spot on a racetrack and then switching the lens to MF, you can take a sequence of photos of cars passing that spot, safe in the knowledge that the AF system won't refocus the lens elsewhere.
Manual focusing is also the only option when the camera can't lock on. Some lenses allow for full-time manual focusing, so you can correct any autofocusing errors made by the camera on the fly, without having the switch the lens to MF mode.
10 reasons why your photos aren't sharp (and what you can do about it)
Metering mode cheat sheet: how they work and when to use them
Best camera focus techniques: 10 surefire ways to get sharp images
Getting sharp images: every photo technique you need to know starting out