Aperture and shutter priority mode are the exposure modes which are controlled by the camera itself. Aperture and shutter priority modes are semi-manual or semi-automatic modes. These features are really helpful as they give you some control over the settings. At the same time they make sure that you get a well exposed image as the camera is engaged in some decision making.
Aperture priority mode- This is also known as ‘A’ or ‘AV’ (Aperture Value). In this mode you have to select the aperture setting, and the camera will decide an appropriate shutter speed in the conditions that you are shooting in. When selecting the aperture one should keep in mind that the camera will be choosing longer shutter speeds, usually the camera sync speed of 1/60th. By choosing F8 you can block out the desired ambient light, giving a photo with better color, contrast and sharpness.
Aperture priority mode helps in controlling the depth of field, which comes in handy in many situations. Aperture priority mode is really helpful in shooting portraits where the background is forced to blur by using large apertures. This separates the subject from its background. For example- while shooting a Black Durgeon, one may notice it disappearing against the deep blue background. This is where the depth of field of aperture mode comes in action and blurs the background, thus highlighting the subject. This also helps while shooting close ups of exclusive marine life as well as underwater landscapes.
Shutter priority mode – This is also known as ‘S’ or ‘TV’ (Time Value). In this mode you have to select the shutter speed, and the camera will decide an appropriate aperture to give a well exposed image. One may shift to shutter priority mode to have more control over capturing the image of a moving shadow. This is a very helpful feature keeping in mind the detrimental effects of camera shake.
Shutter priority mode helps a great deal in underwater photography as the movement of creatures underwater is very swift. Shutter priority mode freezes the motion of a fast moving subject such as sea lion or a shark. This means that if you freeze the image of a fast moving subject by choosing a fast shutter speed, the depth of field will be narrower. Shutter speed also helps in preventing camera shake. Hence, in shutter priority mode you can set the camera at the value where the shake begins and in this way you can blaze away without the fear of camera shake.