taking pictures - aperture focussing?

First of all a small correction...aperture affects the "depth of field" but not the "point of focus".

As for the situation you describe, open your aperture to the maximum, focus on your subject by pressing your shutter button only half way and hold it there. This allows you to move the camera to frame your shot while holding your focus point. Once you have the shot the way you want it, press the shutter button the rest of the way down to take the shot.

Building on the first answer, you may want to switch the camera to manual focus so you can be sure of which part of the scene you are focused on.

Regarding your referenece to "aperture function", some DSLRs have an "aperture preview" setting that lets you see the depth of field when the picture is actually taken. When you are focusing the camera the lens is kept at full aperture (lowest f-number). This maximizes the light going through the camera so that you can compose the photo. When tou take the picture in automatic mode, the camera "stops down" the lens along with changing the shutter speed to obtain the correct exposure. If the scene is well lit, as the aperture may be significantly reduced, which increases the lens depth of field; so what may appear out of focus when you compose the picture could actually end up having a much better focus. The aperture preview allows you to see this effect by closing down the lens to the setting that the camera will select so that you can actually see what will be in focus or not and how much blurring you will get.

The best way to insure a shallow depth of field is to switch the camera either to fixed aperture mode or manual mode so that you can select the aperture that gives you the depth of field effect that you want. In fixed aperture mode the camera will set the exposure by adjusting only the shutter speed. In manual mode you select the shutter speed. You do have to be careful that for the aperture that you select, that you still have sufficient range in shutter speed to get a correct exposure. If not, the other parameter that you can change is the camera's ISO setting to bring it into a good exposure range.

Since taking shots with a DSLR is basically "free", I encourage you to play with the features of your camera. The direct feedback that you get will help you hone your skills quickly.

Often, the default autofocus setting will focus on the nearest object. To focus on someone in the middle of the pack, you have to change the autofocus to use a single, specified point. Then you can put that point on your subject, hold the shutter button halfway down to hold focus while you compose the shot, and press the rest of the way to take the picture. Check your manual to see how to choose focus point and hold focus on your camera. You can also use manual focus, of course.

One of the best sites I've seen for explaining what happens when you change the various settings on a camera is this one

http://dryreading.com/camera/index.html

It takes a moment or two to load.

Click on the various functions and see what happens to the pictures. When you think you've got the hang of it you can try it with your own camera.

The example you've mentioned means you still need to have a very narrow area of your picture in focus, so use f4.8 or something like that. The "narrow" area extends to the front of and also behind where you're focusing.

I think you want to capture a picture like the top one on this page http://tiny.cc/e1A7p or like this one http://blogs.warwick.ac.uk/images/michaelwalford/2...