What is defined as the fraction of the size of the opening in relation to the focal length of the lens?

The fraction of the size of the opening in relation to the focal length of the lens is referred to as the f/stop. The f/stop denotes the aperture setting of a camera, which controls the amount of light that passes through the lens and onto the sensor. It is expressed as an f-number (like f/2.8 or f/16), where a lower number indicates a larger opening, allowing more light, and a higher number indicates a smaller opening, allowing less light. This relationship between the aperture and the focal length is crucial for achieving the desired exposure in photographic images.

Other options pertain to different aspects of photography: shutter speed refers to the duration the camera's shutter is open; aperture priority is a camera mode where the user sets the aperture, and the camera automatically adjusts the shutter speed for proper exposure; exposure compensation allows the photographer to adjust the exposure determined by the camera’s metering system. These elements are important in photography but do not define the specific relationship between the size of the opening and the focal length, which is encapsulated in the concept of f/stop.