Depth of field is the area either side of the focal distance where objects still appear sharp. . Controlling depth of field is a key part of how you interpret the world that you are photographing.
Depth of field is affected by three settings: diaphragm aperture, focal distance and focal length.
A mode is ideal for accurately setting the aperture (and hence depth of field). Your camera will then set an appropriate shutter speed.
With long focal lengths or subjects near to the camera, a shallower depth of field will be obtained. Therefore, before pressing the shutter release button half way, check your balance to ensure you don't move backwards and forwards. This could shift the focal point, resulting in a blurred subject.
Most SLRs have a test depth of field button that will allow you to preview the actual depth of field through the viewfinder. Press the button and the diaphragm will be stopped down to the value set in your camera. Note: the image through the viewfinder will probably appear darker. Check your manual to see where this button is on your SLR.
To obtain very large depths of field, stop the diaphragm down to f/11, f/16 or even f/22. The exposure time will be increased.