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A Photography Lesson: The Exposure Triangle

Are you interested in learning a bit about photography? Read on for a lesson in the exposure triangle.


First things first...You need to know a few key words to understand photography. A big one is "Exposure". A photograph's exposure determines how light or dark an image will be. This is determined by the Exposure Triangle. Sounds fancy right?! The exposure triangle is your cameras Aperture, ISO and shutter speed.

To learn how to have the proper exposure is a lot like collecting rain in a container. You can't decide how much it rains right? But you can decide how big your container is, how much time you keep your container in the rain, and the amount of rain you want to fill up in your container. The exposure triangle of aperture, ISO, and shutter speed corelates to the width, time and quantity of our rain container. If you leave your container out too long you'll end up with an over exposed image (or too bright). If you don't leave it out long enough it will be under exposed (or too dark). The key is that there are many different combinations of width, time and quantity that will achieve this. The cool thing is that you can adjust one setting to compensate for another. Each setting effects the other so each change you make requires you to consider the adjustment needed for the exposure triangle to end up with a properly exposed image. Another great example is how our eyes work similar to a camera. I'll save that for another post!

So, lets dig in a little further to understand each setting. Aperture, ISO, and Shutter Speed each control exposure in different ways.

Aperture: controls how much light can enter your camera (how wide the shutter opens)

Shutter speed: controls the duration of the exposure (how fast the shutter opens and closes)

ISO speed: controls the sensitivity of your camera's sensor to light

Free tidbit: ISO stands for the International Standards Organization. People often say ISO as a word but it's actually I-S-O. ISO is a system for measuring camera sensitivity. These are the ISO 12232:2006 standard which report ISO speed ratings, ISO speed latitude ratings, standard output sensitivity values, and recommended exposure index values, for digital still cameras. There were a couple other organizations prior to ISO until the late 70's.

So, you can use a variety of combos to get to a "correct" exposure. You just have to learn what trade-offs to make. Each setting will impact image properties.

Aperture affects depth of field

Shutter Speed affects motion blur

ISO speed affects image noise


A camera's shutter determines how long the camera sensor will be open or closed to light from the lens (how long light enters the camera).


A camera's aperture controls how big the area light can pass through your camera lens is. The aperture has a f-stop value which can be a bit confusing because the smaller f-stop is the wider open (the more light allowed in). For example f/1.8 is much wider than f/16. In tog slang, when someone says they are "stopping down" or "opening up" their lens it means increasing and decreasing the f-stop.

1/800 sec. (shutter speed) f/4.5 85mm (aperture) ISO 200 (ISO)

Do you see how more of the bench is in focus with a smaller aperture (higher f-stop value = smaller opening)

1/4000 sec. (shutter speed) f/1.8 85mm (aperture) ISO 200 (ISO)

Do you see the difference in the depth of field? This was shot "wide open" at f/1.8.


The ISO speed determines how sensitive the camera is to light. Like shutter speed, it also correlates 1:1 with how much the exposure increases or decreases. Most often you want a low ISO speed since higher ISO speeds dramatically increase image noise. So, you usually only increase ISO speed if you can't use the aperture and shutter speed you want without adjusting it. When I used my Canon A-1 film camera I would buy the film at the ISO speed I wanted. I didn't adjust it on the camera. So, I had to use the whole role and plan my shoot accordingly. Times have changed!

I hope this lesson helped you! There is so much more to dive into but I hope this helped give a starting point to understand the basics of how the Exposure Triangle works!

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