Autofocus is a fantastic invention which has simplified photography and greatly reduced the chances of ending up with a blurry image. However, it isn't perfect, and there are times when you'll need to give your camera a helping hand to get a sharp picture.
Focus lock is a handy tool for doing just that. It allows you to focus on the most important part of your scene and then hold that focus while you recompose the shot, ensuring your final image has the correct parts in focus.
There are a couple of different ways to use focus lock, depending on the situation.
Locking Focus for a Single Shot
This is the most common use of focus lock, and the simplest to perform. It's useful for shots like the following, where you have a subject placed off-centre in the frame, in front of a distant background.
If you compose your shot like this and focus as normal, your camera will try to guess what needs to be sharp. Most cameras consider the centre of the picture to be the most important part, and focus on it. This will result in the background being sharp, but the woman in the foreground being blurry.
Clearly this isn't what we want; we want the woman to be in focus seeing as she's the most important part of the shot. This is where focus lock comes in.
To use it, point your camera at the subject and half press the shutter button. You should hear a beep and see a light come on in the viewfinder to let you know the camera has focused. The focus will now remain locked while your finger is still holding the shutter button half down.
With the focus locked you can now recompose your shot before taking the final photo. In our example image, we'd bring the camera back to the composition we want and press the shutter button down fully. This would leave us with the same picture before, but with the woman rather than the background in focus.
This sort of focus lock is quick and easy to use, and gives you a lot of flexibility without having to mess around with settings like autofocus points. However, it only lasts for one shot - as soon as you take a photo the focus is lost and you need to repeat the process. If you'd like to re-use your focus point for many shots, you can use the next method.
Focus Locking for Multiple Photos
If you want to take several photos all with the same focus distance, normal focus lock won't work because it doesn't "remember" its settings between photos. However, if you own a digital SLR there's a simple trick you can use to keep your focus constant for as long as you'd like.
Start by focusing your scene using your camera's autofocus as normal. Once you're happy, flick your lens into manual focus mode (most lenses have a switch on the side). This disables the autofocus feature and keeps the focus distance fixed where it is, ensuring that all shots are focused the same. When you're done, simply switch back to autofocus mode.
This only works in situations where the subject is always at a constant distance from the camera, such as in still life or landscapes. It's also useful during some action sports such as motor racing, where the cars pass by on roughly the same bit of track each time.
Focus lock is a really useful tool and one that I use on virtually all my shots to ensure the right part of the scene is in focus. It takes no time to learn and you'll soon find it's an invaluable technique for improving both the composition and clarity of your photographs.