A lot of my friends simply put their DSLR to Program mode and shoot. And when asked why don’t you use other modes to shoot the reply was the photos taken in other modes were not sharp or that the exposure were awful. And after further probing, they do not understand the correct operation or under what circumstances should they use the Aperture Priority mode or Shutter Priority mode.
As the name suggest, when this mode is selected the user will set the aperture and the camera will determine what shutter speed to use according to the lighting condition.
This mode is extremely useful when you have abundant lighting, or when you want to control the exact depth of field of your photo. If lighting is poor you may end up with a rather slow shutter speed and result with a blur photo.
When to use Aperture Priority
For example, when I was in Inner Mongolia I came across a nice scenery and I wanted to capture that vivid image with my camera. The lighting was bright and sunny. And I wanted good depth of field to capture the foreground and at the same time a sharp background. I chose Aperture Priority to lock in a small aperture and let the camera compensate the exposure with shutter speed because changes in shutter speed will not alter or affect the photo that I had in mind much.
This mode allows user to set the shutter speed and let the camera determine the aperture according to the lighting condition. This mode is ideal when you need to capture fast moving object and that depth of field is not of a concern. However, just like in Aperture Priority, when lighting is poor you will result in getting an under exposed photograph.
When to use Shutter Priority
For example, during the MotorGP, every motorbike travelled at a very fast speed, the lighting was bright and sunny, and depth of field was not really as important. After considering these factors, Shutter Priority was chosen. I would then set a shutter speed whereby I know the photographs will yield a sharp image and let the camera choose the corresponding aperture.