The way most modern cameras is set up is that when you half push the shutter button down the camera automatically focuses and takes an exposure reading at the same time. Then you push the shutter all the way down to take a photo.
This way of doing things is OK for some situations but for others it's very annoying. Take this photo of a sunset of Wat Mahathat in Ayutthaya, northern Thailand. There've been a few sunset shots over on the Flickr group so I thought I'd get in and post my own!
Now in a situation like this, where the focus is on the spire of the Wat (temple) in the foreground, we're presented with a problem. The focussing squares of many modern cameras are in the middle of the frame. So if we want to focus on the Wat we have to move the camera to the side, position the focussing square over the spire, half push the shutter button down, and then re-frame the picture. And as soon as we take a photo that focus is lost. To take the same shot again (which you would do because the sun is moving and you might want different variations) we then have the move the camera back and re-focus it on the Wat every time we take a shot. Very annoying.
In the old days what I always used to do was focus once on the Wat and then put the lens on to Manual focus. That way every time I pushed the shutter button I wouldn't have to worry about postitioning the focus square back over the Wat again. The focus was locked.
With my digital camera though I do it differently. In the custom functions of many cameras you can change the button that you use to focus the camera with. One of the options is to change it to the Automatic Exposure Lock button, represented by a * on the Canons and AEL on many other brands. It sits in a very comfortable position right where your right thumb rests naturally, on the top right hand side of your camera just in front of the LCD if you have one. So in order to focus all you do is push down with your thumb. It couldn't be easier.
So now when I'm taking a photo like the one above I use autofocus on the Wat using the * button and then take my finger off the button. As long as I don't push the * button again the focus is locked. So I can re-frame my picture and take lots of photos and the focus won't move.
The advantage of using this method over the 'switch the lens to Manual Focus mode' way of doing things is that if something catches your eye in the foreground and it's moving pretty quick, you don't have to remember to switch your camera back to autofocus. You just point your camera at the new photographic subject and push the * button gain.
Try it, I guarantee that once you get used to it it's an addictive way of focussing your camera. The only down side with it is if you give your camera to somebody to take your picture, they almost never manage to focus the camera! Telling them to push the * button to focus and then the shutter button to take the photo seems to confuse most people and you come away with a whole bunch of blurry photos of yourself. I speak from experience!
Have a great weekend and don't forget to post some shots or ask some questions of the Flickr group.