Aperture and shutter speeds combined

Manual Techniques - Aperture and shutter speeds combined

If you are not wanting to alter all of the settings yourself you can choose either to have aperture priority (depth of field) or shutter priority.  If it is the depth of field that you need to control but the shutter speed is less of an issue for example if the camera is tripod mounted for a macro shot then use the aperture prioirty and choose your f stop.  The camera will then automatically choose the best shutter speed for your image.  

If it is shutter speed that will make your image work for example you need a fast shutter speed to freeze an action shot then select shutter priority.  The camera will now select the best aperture for your image (depth of field).

The diagram above shows the affects of adjusting the shutter speed on the aperture and vice versa.

Collins Digital SLR Handbook - John Freeman