Typically, a modern digital camera offers a variety of shooting modes: automatic, program, shutter-priority, aperture-priority, manual and bulb. These all offer advantages in different scenarios.
Fully automatic shooting
Setting fully automatic shooting “A” pretty much hands over all control to the camera, which then becomes a point-and-shoot device with most of the options disabled. This is a very useful, foolproof mode if you are on holiday and give the camera to someone for a moment to take a holiday snap of you. However, if you are reading this you will probably wish to read on and use a mode that gives you a bit more control.
In program mode “P”, the camera sets both the shutter speed and the lens aperture according to the light reading and a pre-defined program. Some cameras will take the lens focal length into account and also adjust the ISO as needed. All the camera’s advanced controls are usually available including exposure modes and exposure compensation. Many photographers look down on this mode as amateurish but in my experience it works very well in many circumstances and should not be overlooked.
Aperture-priority and shutter-priority allow more creative control than program mode. In aperture-priority “Av”, the photographer sets the aperture and the camera sets the shutter speed according to the light reading. As aperture strongly influences depth of field, this might be thought of as the “depth of field mode”. For example when taking a portrait we may wish to focus on the person and set a wide aperture such as f/1.4 to blur a distracting background, accepting the camera’s choice of shutter speed as long as it is reasonable. Similarly for landscapes we may wish to set a narrow aperture such as f/11 to increase depth of field. If the camera selects a shutter speed that is unsuitable, perhaps too slow, the ISO can be adjusted to change the camera choice of shutter speed.
In the portraits below, the first was taken at f/16 and has a very distracting background. By opening up the lens to f/1.8 for the second image, the background was thrown out of focus, giving a much improved portrait.
Shutter-priority “Tv” is preferred when we are more concerned with freezing motion, or conversely introducing some motion blur. For example when photographing a bird in flight we may wish to set a shutter speed of 1/1000 second to freeze the motion and let the camera determine the aperture needed for proper exposure.
For a detailed technical comparison of Tv and Av modes, see: Tv-Av comparison
For more discussion about setting shutter speed, see: Shutter
In manual “M” mode, the photographer sets both aperture and shutter speed, allowing total creative control of depth of field and the amount of motion blur. Typically, we would set the aperture and speed that we want and which seem reasonable for the lighting conditions and then take a light reading, adjusting aperture or speed as needed for a correct exposure. Using manual mode requires a little more practice than the semi-automatic Av and Tv modes but it encourages the photographer to think more about the creative process. Some modern cameras allow the ISO to be set to automatic while in manual mode, allowing the photographer to set aperture and shutter speed while the camera adjusts ISO to achieve correct exposure.
In bulb “B” mode, the shutter remains open for as long as the shutter button is held down. This is useful for night scenes, fireworks and so on that need long exposures.