ISO is a standard or light sensitivity set by the International Organisation for Standardisation.

ISO controls how sensitive the cameras sensor is to light; the lower the ISO, the less sensitive and vice versa. However, when you use higher ISO, the photos taken could look grainy or ‘noisy’ and the overall quality would decrease – as shown below

High ISO is only really necessary in very dark settings as it will make the photos brighter, yet it sacrifices the photos quality. Because of this, you should generally try to avoid using an ISO that’s too high, and find other ways to brighten your photos, such as shutter speed.

The photos captured above show the same general subject, however each taken with different ISO, the darker photos are more sharp however clearly are too dark, but the lighter photos are less sharp and too bright – more detail below – , so be sure to find the ISO right for the environment you’re photographing.

This is a larger example of my ISO 100 photo, and as you can see there is no visual noise, the chairs are in sharp focus, although they may be hard to see well due to lack of light.