Light Factor

Besides changing the shutter speed to manipulate light as it enters the camera, you can adjust the aperture, the opening through which light enters the camera. 
Like an eye, the aperture expands or contracts according to how much light you want. 
The aperture is adjusted according to a scale of f-stops. The lower the f-stop, the wider the aperture opening and the more light is allowed in. 
An f-stop number is universal from one lens to another.