Melissa Gaston: Find The Light


Photography 101: The Exposure Triangle

Understanding the Interdependence of Aperture, Shutter Speed, and ISO

The word “photography” comes from the Greek words “photo,” which means light, and “graph,” which means drawingit literally means to draw with light. This blog is called Find the Light, because photography is all about understanding how to use the light that’s available to create the picture you want.

There are three variables that work together to create a properly exposed photograph: aperture, shutter speed, and ISO. (Click here for a link to my glossary of photography terms.) Together, these three items make up the Exposure Triangle.

Each camera has a built-in light meter. Most of the time, it accurately measures the light around you and determines best combination of aperture, shutter speed, and ISO for those conditions. (There are circumstances where your camera’s light meter gets it wrong, but that’s a topic for another blog post.)

You can manipulate each of these variables to achieve different results:

  • To control how much of your photo is in focus, your aperture setting is the most important setting.
  • To control how much motion blur there is in your photo, or how sharp fast-moving subjects are, your shutter speed is the most important setting.
  • To control how grainy (or clean) your image will be, then you need to focus on your ISO.

However, as aperture, shutter speed, and ISO work interdependently, gaining a benefit in one area (such as a fast moving subject in focus) may require you to give up something from one of the other variables (often a grainier photo from a higher ISO).

As an example, let’s say that you are shooting a landscape and you want the entire scene in focus. That will require that you use a higher f-stop number, such as f/22. This means that the opening on your camera lens will let in significantly less light than if you had set your aperture to f/4.  In order to use f/22 and still adequately expose your photograph, you will need to increase the amount of light coming to the sensor by: 

  • decreasing your shutter speed (leaving that shutter open longer and raising the risk of motion blur) or
  • increasing your ISO (and increase the graininess in your photo).

The reality is, most of the time we try to find a happy medium.

Don’t panic if this isn’t making complete senseI’ll be writing blog posts on each item in the exposure triangle in the near future. For now, I just wanted to give you a quick heads up that these items are interdependent.

Here’s a quick video that illustrates how the interdependence looks on a camera:

Want to know more about the individual parts of the exposure triangle? Start with Photography 101: Understanding Aperture.