Exposure determines how light or dark an image will appear when captured by your camera. Exposure is controlled by three factors that can be selected on your camera: shutter speed, aperture, and ISO. These three factors and are commonly depicted as sides of the “exposure triangle.” Shutter speed and aperture control how much light is exposed to the camera’s sensor. The ISO controls the light sensitivity of the sensor.
What is a stop? A stop is the change in the amount of light reaching your sensor or the light sensitivity of the sensor and is a factor of 2 increase or decrease in the amount of light or sensitivity (i.e., 2^n). So, a 1 stop increase results in twice (2^1) as much exposure and a 2 stop increase results in four times (2^2) as much exposure. A 1 stop decrease results in half (2^-1) as much exposure and a 2 stop decrease results in a quarter (2^-2) as much exposure. Stops can also be measured in fraction, such a 1/2 or 1/3 stop.