Exposure Digital Field Guide

Chapter 5. ISO

What is ISO, and how does it affect exposure? Changing the ISO doesn't change the amount of light reaching the camera's sensor; rather, it changes the sensitivity of the sensor to the light that is reaching it. In simple terms, the ISO defines how sensitive your camera's sensor is to light. Understanding what ISO is and when to change it is very important, yet for many photographers. Using the wrong ISO can have devastatingly negative effects on your image; while picking the right one can help you capture images better than ever before. Understanding the relationship between the ISO and light sensitivity, digital noise and more importantly, how to reduce it are all covered here along with the best times to use the high ISO capabilities of your camera.

To get this photograph of Phil Lesh, bass player for the Grateful Dead, I made sure I was using a shutter speed fast enough to freeze the action and still get a good exposure. This meant shooting at 1/200 second, f/2.8, ISO 1600.

Understanding ISO and Light Sensitivity

To discuss ISO properly and the light sensitivity of a digital camera's sensor, we have to go back to the days of film, and more importantly, we need to discuss the different film speeds and their sensitivity to light. The ISO (International Organization for Standardization) rating that replaced the older ASA (American Standards Association) rating measures ...