Your camera has a tool that controls its sensitivity to light. The function that controls this is called ISO – and if you want to win your next trivial pursuit night it stands for International Standards Organization. As you select a higher numerical value for your ISO your sensitivity increases. So 100 is a low sensitivity and 3200 is quite high. Essentially the higher the ISO the faster your shutter speed can become.
Unlike aperture and shutter, ISO has no corresponding creative capacity. Aperture controls depth of field and your shutter controls how crisp movement is in your photograph. But ISO, at its core, is a balancing act between the picture being bright enough and the degradation of the image.
Just like how turning up the volume on a radio will increase the static you hear, increasing your light sensitivity will increase visual static in the photograph. We call this digital noise and it manifests as dots of color appearing in your photograph that were not actually there. These dots lower the clarity of the photograph.
If we examine the dark areas of this photo, or zoom in on any of the details we’ll see clean black and clear delineation between subject and background.
1/60, f/3.2, ISO 100
Now look at those same areas – Loss of detail in the subject, blurred edge where subject meets background.
1/200, f/4, ISO 1600
Now we have a total loss of quality. Detail is missing, loss of delineation.
1/800, f/6.3, ISO 12,800
So why would we increase our ISO? Look at the shutter speed in the previous photographs. It increases dramatically as we become more sensitive to light. And that is the critical advantage of ISO: increased shutter speed. For this reason we call ISO a crutch for your shutter speed.
There are two types of blur in photography: the movement of the camera or the movement of the subject. Either of these can be eliminated by a fast enough shutter speed. If I need to shoot a fast shutter speed in low light I need to either gather more light through the lens (open my aperture) or increase my sensitivity to the light I have available (ISO). When our aperture opens as far as it can go, ISO is our last best hope.
Here’s one more example of this concept: