Exposure is all about making sure the sensor gets the right amount of light. Without camera exposure controls, pictures taken at night would be pitch black, and those taken in bright daylight might be burned out. There is actually a fairly narrow ‘window’ of light intensities which sensors can record, and real-world lighting conditions vary […]
This is a basic principle of exposure control. If you adjust the camera's shutter speed, you can compensate for this with an adjustment to the lens aperture and vice versa – a 'reciprocal' adjustment that maintains the same overall exposure.