Every photograph is an exposure, created through the use of three controllable factors: Aperture, Shutter Speed and ISO. With the camera set to the correct exposure, if you change one of these settings, you will need to change one of the others to keep that correct exposure. We use the word exposure because when we create a photograph we expose the camera’s sensor to light. Although in digital photography the ISO does not technically control sensitivity to light, it is a useful term to simplify what the ISO achieves.
Exposure is measured in ‘stops’ each stop is half or double the amount of light of the previous number. For example f/4.0 is half as much light as f/2.8, 1/125th second is twice as much light as 1/250th second, ISO 400 is four times as much light as ISO 100. – on a digital cameras this is broken down further into thirds of a stop, each click on the camera control dials changes the exposure by 1/3 of a stop.