Your digital camera and SLR camera's autofocus lock explained

All autofocus cameras use either subject contrast or infrared detection to determine the subject's distance from the camera. The trouble is, although technically highly advanced, they can still easily be fooled.
Your autofocus camera will have either passive or active autofocus, in simple terms passive uses electronics that can measure subject contrast to determine the distance, while active fires out an infrared beam to measure the distance. Unless the camera is one of the newer models with a wide area AF or multi point focus, the subject has to be in the centre of the frame and here's the downfall, and one reason why you may have got back blurred photos from your holiday or special occasion. If you take a photo with the subject off-centre the camera will focus on whatever is in the background. If this is a similar distance your shot will be safe, but often it's much further away and the camera adjusts focus to the background. The result is a blurred subject in front of a razor sharp background.

The way round this is to use the camera's focus lock feature. Most models have one which is activated by half pressing the shutter release. A viewfinder indicator lights up to tell you that the focus is locked. When this happens you keep the shutter depressed and recompose with your subject in its desired off-centre position.

You can also use the focus lock to ensure the subject has focused on exactly the right point of the subject. This is important for close up photography where it may not be possible to get the whole of an insect or flower in focus. In such cases pick the area that's most important and focus there. Where possible set a small aperture to ensure maximum depth-of-field. We have more details of depth-of-field here.

This photo shows what normally happens when you take a picture with the subject off-centre. The left hand shot has the subject blurred, but the background hedge is sharp. Using the focus lock on the statue has ensured it comes out sharp.

Sometimes there's not enough depth-of-field to allow all the elements within a photograph to be sharp. The left hand shot shows the camera has focused on the joins of the tap, but the head is unsharp and this is very noticeable. By locking focus on the tap head the shot looks sharper, even though the connecting nut and rag are now blurred.

The camera has focused on the background trees. A quick focus lock on the flower ensured that it came out sharp against a very blurred backdrop.

You can also use the focus lock when shooting through a window to prevent the camera from being fooled and focusing on the glass. If you're on a coach lock the focus on the furthest point you can find and the recompose through the window.

Beware some cameras lock the exposure at the same time as the focus. If so you have to be careful when shooting in tricky lighting because the exposure on the area that you are focusing on could be very different to the actual composition that you shoot. Some more advanced SLR cameras have an option to select the mode used for the AEL button so you can have focus lock and/or exposure lock.