ISO is a foundational principle for photography. It is one of the three elements of theExposure Triangle, along with Aperture and Shutter Speed. Exposure is all about lighting, of course, and how much of it our film or digital sensors are exposed to. And precisely controlling that amount is crucial to creating great pictures. When you’re fully in Manual Mode, you have control over all three elements.
But even pros often shoot in Aperture Priority (Av) or Shutter Priority (Tv) mode, leaving the other and ISO up to the camera to decide on. Unless the lighting is extremely harsh or dark, the camera is usually pretty intelligent about choosing a good ISO setting for a given scene. And of the three, ISO may be the setting you adjust the least. But when you do need to, you’ll need to have a solid understanding of what it’s doing and what you’re trying to achieve.
What is ISO?
The term “ISO” hearkens back to the old days of film photography. Anyone who has shot film will know or remember that it comes in ISO ratings that describe how sensitive the film is to light. And the higher the ISO number, the more sensitive that film is. And the darker the environments it can be used in.
Like crop factor, ISO is a holdover from the days of film that still describes a digital concept nicely. Most digital cameras have a minimum ISO value of 100 or 200. This is meant to be used in bright lighting in order to achieve clarity. The sensor is the least sensitive to light – which may seem strange. After all, don’t we want more light as photographers?