Beyond Auto Mode: A Guide to Taking Control of Your Photography

Chapter 2: ISO

In photography we use the term ISO to indicate how sensitive film or a digital camera is to light. If you use film, you choose a film speed (such as 400) based on the lighting conditions. Digital cameras are no different—you choose ISO based on the amount of light in the scene. The higher the ISO, the more sensitive your camera is to light; the lower the ISO, the less sensitive it is.

Put another way, the ISO refers to the amount of light the film or camera sensor will receive based on the setting you select. As you increase film speed or your ISO, the film/sensor is able to “absorb” more light. Essentially, more light is available for you to make your exposure. At ISO 50 less light is available, while at ISO 400 more light is available.

The term ISO refers to both film speed in analog cameras and light sensitivity in digital cameras.

With film, we must evaluate the light in our scene when choosing which film to use (ISO), and learning what film is best for each lighting scenario is necessary to make the appropriate choice. A sunny day, for example, might call for a film speed of 100, while a darker scene would find 1600 more appropriate. ISO 400 is a good catchall speed if you are unsure what your selection should be.

With a digital camera, the same principles apply. We choose a lower ISO of 100 in a very bright setting, and move to an ISO setting of 1600 ...