When to use ISO
Let’s say you’re shooting a football match in the winter, where you know you need a shutter speed of 1/500 sec:
When it kicks off at 3pm, the daylight is still quite good and you can shoot with an exposure of 1/500 sec at f/4.
By half-time, the light has fallen by 1EV so that you have to increase the lens aperture to f/2.8 to be able to get the same shutter speed.
During the second half of the match, though, the light drops a further 1EV and you’ve already reached the lens’s maximum aperture. The solution is to increase the ISO instead, from ISO200 to ISO400.