How did you get on with Your Challenge from the Depth of Field post? (You were to take of a photo with a shallow depth of field where there were multiple objects, but the main subject was not in the centre.)
Our cameras are pretty good at autofocusing. They have 20, 30, 60, or more focus points. Focus points are the areas in the frame the camera uses to lock onto the subject. You can see them in the viewfinder as squares arranged in a pattern.
The problem is that there are so many focus points the camera could use, but the technology hasn't been developed yet where the camera can read your mind to know what you specifically want to be in focus. While the autofocus system does a good job guessing it often get it wrong, particularly where there are multiple objects at different distances in the frame.
It may seem like a waste, but let's set our cameras to only use the focus point in the centre. Exactly how you change your focus points on your camera is something you are going to have to ask Google. Generally speaking, on Canon cameras, you press a button at the back of the camera (usually top right), and roll the dial until you get what you want. Or on some newer cameras you can do this on the screen.
Once you have the camera set to only use the centre focus point you need to adopt a technique called Focus and Recompose to make sure your subject is in focus.
Focus and Recompose is a powerful way of giving you control and works like this: looking through your viewfinder, put the subject, or the area you want in focus in the exact centre of the frame. Half-press the shutter button. This should get focus. Now, while still keeping the button half-pressed, frame your composition as you would like it. Full-press the shutter button to take the shot.
Got that? Half-press to get and lock focus, then move to compose your photo.
If you found the last Your Challenge challenging, or found other workarounds (aka cheats), you may want to practice the Focus and Recompose technique. It really is one of the key skills that many - if not most - pro photographers use.
Unless you build up the muscle memory for Focus and Recompose, it can be hard to remember to where you are supposed to get focus, particularly in rapid-fire situations. One of the pitfalls to look out for is where you have two people / subjects. With the centre focus point, it may try get focus between them, which would be bad.
Take the opportunity to learn about your own camera's focus modes and capabilities. How many focus points does your camera boast? What buttons do you need to press to change which focus points are used?
Practice the Focus and Recompose technique. With your camera set to use the centre focus point, take a photo where your subject is partly obscured by some other object close to you. And don't forget the composition non-rules.