The amount of light per area at the sensor is directly proportional to the area of the aperture and inversely proportional to the focal length. That's why we us

The amount of light per area at the sensor is directly proportional to the area of the aperture and inversely proportional to the focal length. That's why we use the f-number, which is a ratio of focal length to aperture diameter. Please note that the f-number doesn't indicate total light hitting the sensor. It indicates total light per unit area at the sensor. Thus, for the same f-number, a larger sensor will gather more light. All of this info can be found in the wikipedia page: https://en.wikipedia.org/wiki/F-number https://en.wikipedia.org/wiki/F-number