Three Gateways for Light (Shutter, aperture, ISO speed)
To demonstrate the three gateways, let's use water to represent light
Sensitivity - Cup size
Adjust the three gateways to obtain a correct exposure
The camera has three gateways to adjust the amount of light. It's like adjusting the amount of water flowing from a faucet. If you open the faucet all the way, more water will flow and the cup will fill up faster. However, if you open the faucet only a little, less water will flow and it will take longer to fill the cup.
This faucet adjustment is like the camera's aperture. And the length of time to allow the water to flow is the like the shutter speed. The size of the cup is like the sensor's sensitivity. The bigger the cup, the longer it will take to fill it with water (lower sensitivity). The smaller the cup, the faster it will take to fill it (higher sensitivity). The amount of light received by the camera is also called the "exposure."