4. How Aperture, Shutter & ISO Work together.

Now that you understand the three building blocks of exposing the photograph, you need to see how they work together to gather the correct amount of light. Each scene you photograph needs a decision from you or your camera about the exposure settings to use and to do this you utilise the shutter, the aperture and the ISO.
An important concept to understanding how aperture, shutter and ISO work together is that each shift from one value to the next is called a ‘stop’ and each stop lets in half as much OR twice as much light as the previous stop. Some cameras let you select a half or a third stop but for the purposes of this explanation we will use only one stop.

In this table below column1 lets in 1 stop more light than column 2 and column 3 lets in 1 stop less light than column 2. The ISO is the same principal with column 1 being twice as sensitive as column 2 and column 3 being half as sensitive as column 2.

Shutter: 1/60th lets in twice as much light as 1/125th and half as much as 1/30th. Aperture: f5.6 lets in twice as much light as f8 and half as much as f4. ISO: ISO 100 is half as sensitive as ISO 200 and twice as sensitive as ISO 50.
By making changes to each of these settings you can achieve a correct exposure but you can decide which element is the most important and use that instead. An example will make that clearer.
If you make a change to one element, say the ISO, adjusting by 1 stop from 100 ISO to 50 ISO, then the shutter or aperture has to be adjusted accordingly to compensate for the loss of 1 stop of sensitivity to light, adjusting either the shutter or the aperture. Lets assume your camera is set to ISO 100, and currently your camera is set to f5.6 at 1/125th.

The table below shows you how you can achieve the same correct exposure by adjusting the shutter, aperture and ISO.