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Aperture and shutter speed
A digital camera may have manual and/or semi-automatic modes that let you control aperture and shutter speed. Together, aperture and shutter speed determine how a photo is exposed.
The faster the shutter speed, the larger the opening of the lens (ie. smaller aperture number) and visa versa.
Aperture-priority (AV) mode
When using aperture-priority, also known as Av mode, you manually select the aperture and the camera automatically sets the shutter speed.
The aperture is the opening in a lens that admits light onto the camera sensor and is varied by changing the size of the opening. It's diameter is stated as an f-stop. Aperture-priority mode is used when you want to control depth-of-field.
Shutter-priority (TV) mode
Shutter speed relates to how long the camera's shutter stays open after you depress the shutter-release button. Shutter speed is measured in seconds, such as 1/125th. A slow shutter speed produces a longer exposure time.
For shutter-priority mode (Tv), select the shutter speed and the aperture adjusts automatically. Use shutter-priority mode when you want to control action in a photo either to freeze the action of fast moving subjects or to create a sense of movement.
