We’ve come to the final corner in the Exposure Triangle, ISO, and it’s the easiest corner to understand.
ISO was a property of film that determined how sensitive it was to light, also referred to as film speed. There are a variety of speeds ranging from a fast film (say ISO 50) to a slow film (say ISO 1600) . In order to use a fast film you need a lot of light, e.g. bright light on a beach in the afternoon. That also meant in darker conditions you would need to have a wide aperture or a longer shutter speed to get the correct exposure. However, a fast film will react to light much quicker and won’t need as much; this makes it useful for night photography. The problem with the higher ISO films is that the images lost image quality. This was due to the grains of emulsion used on the film and resulted in a ‘grainier’ look.
Now with modern technology we are able to change the ISO on our digital cameras. The image quality at high ISO will vary on the ability of the camera but most recent digital cameras can easily shoot at ISO1600 or higher. Just like with film, if you choose a low ISO (say 100) you will need a lot of light to get the correct exposure. However, if you shoot at high ISO (say 1600+) there will be a reduction in image quality as ‘noise’ will be more evident.
ISO is similar to shutter speed as it correlates 1:1 with how much the exposure increases or decreases. However, unlike aperture and shutter speed, a lower ISO speed is almost always desirable, since higher ISO speeds dramatically increase image noise. As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren’t otherwise obtainable.
Below are examples of different ISO’s with the same Aperture of f5.6.
As the ISO increased you can see the increase in the digital noise.