Final answer:
The light-gathering ability of a camera lens is specified by its aperture, defined by the f-number, which is the ratio of the lens's focal length to the diameter of the aperture. Lower f-numbers correspond to larger apertures and better low-light performance, while higher f-numbers indicate smaller apertures suitable for bright conditions.
Step-by-step explanation:
The light-gathering ability of a camera lens is specified by its aperture. The term f-number or f/# represents the light per unit area reaching the image plane and is calculated by the ratio of the lens's focal length (f) to the diameter (D) of the aperture that controls the entry of light. As the f-number decreases, the aperture is wider, allowing more light to reach the image sensor, which is beneficial for low-light photography. Conversely, a higher f-number indicates a smaller aperture, which is useful in bright situations to prevent overexposure. This concept is also relevant when focusing light into optical fibers, where a similar principle of light acceptance angle and aperture size governs the efficiency of light transmission.