Aperture and exposure
The aperture in a lens – also known as the “diaphragm” or “iris” – is an ingenious piece of mechanical engineering that provides a variable-size opening in the optical path that can be used to control the amount of light that passes through the lens. Aperture and shutter speed are the two primary means of controlling exposure: for a given shutter speed, dimmer lighting will require a larger aperture to allow more light to reach the image sensor plane, while brighter light will require a smaller aperture to achieve optimum exposure. Alternatively, you could keep the same aperture setting and change the shutter speed to achieve similar results. But the size of the opening provided by the aperture also determines how “collimated” the light passing through the lens is, and this directly affects depth of field, so you’ll need to be in control of both aperture and shutter speed to create images that look the way you want them to.