The Exposure Triangle

Combining the Three Exposure Controls

If you have being paying attention you will have noticed that shutter speed, aperture and ISO are all adjusted by stops (where altering exposure by a stop either doubles or halves the amount of light reaching the sensor). 

The practical upshot of this is that all three have a reciprocal relationship. Adjust one and, in order to maintain the same exposure, you will need to adjust one or both of the other two by the same number of stops. (In Manualexposure mode particularly. In the automatic and semi-automatic modes this is done for you – though ISO needs to be set to Auto ISO for ISO to alter too when shooting in Program, Shutter priority or Aperture priority.)