The Exposure Triangle

The Exposure Triangle

by William Lulow

Note: Thanks to my student, John Baiata who inspired this article.

I was going through the subjects and articles I usually use to teach the concept of getting the correct exposure for one’s digital images, when I discovered that I really hadn’t written many articles about what has come to be referred to as “the exposure triangle.” This is basically, the combination of sensor speed (ISO number), shutter speed and aperture, all of which control the amount of light that creates the image. At least all the light that the camera controls. You can add artificial light to most any scene, but that’s another topic for another article. (I have written extensively about artificial light in other articles. So, if you are interested, you can search the archives. I also re-publish them from time to time).

So, these three elements work hand in hand to create the initial exposure. Here is a chart of the settings needed to make the SAME exposure:

The following settings will yield the same basic exposure:

Aperture                                Shutter Speed                                 ISO

f/2.8                                        1/125^th second                                 100

f/3.5                                        1/60^th second                                    100

f/4.5                                        1/30^th second                                    100

f/5.6                                        1/15^th second                                    100

f/8                                            1/7^th second                                      100

f/11                                         ½   second                                        100

f/16                                         1 second                                            100

One glance at this chart may be confusing at first, so keep in mind that each of these settings produces the exact same exposure. So, keeping the sensor speed (ISO) the same, you would need to change both the shutter speed as well as the aperture to create the same exposure.

This presents many different choices right from the beginning. You can shoot at a fast shutter speed, but if you do, you will have to open up your lens to its widest aperture. Or, you can use a very small aperture, in which case you will have to slow down the shutter speed considerably to let in the right amount of light. Each of these settings will dictate how you must approach your shot making. If you use a slow shutter speed to obtain the greatest “depth-of-field” available from a very small aperture (f/stop), you will probably need to put your camera on a tripod if you want a clear image. Conversely, if you wanted very little depth-of-field so that your background would be blurry and indistinct, you would need to open your lens and shoot at a faster shutter speed.

Now each lens aperture “stop” allows a certain amount of light to hit the sensor (or film). Going from f/2.8 to f/3.5 reduces the amount of light by “ONE STOP.” By the same token, going from a shutter speed of 1/125th of a second to 1/60th of a second allows for an increase of one stop of light to enter. So, the amount of light you are allowing to hit the sensor follows an opposite kind of ratio. If you close the aperture by one stop, you have to increase the exposure time one stop to arrive at the same exposure.

Every step of the way requires the photographer to ask “what kind of image do I want to make”? Once you can answer this question, you are on your way to making the kind of image you really want. So the key is to answer the question first instead of just holding the camera to your eye and making an acceptable image.

Here are two almost identical images. See if you can note the differences:

Also, see if you can guess which was shot in AUTO mode and which in MANUAL mode. Differences in the look of each image is dependent on not only which mode you use, but also on what you want the final image to look like.

Okay, enough with the suspense. The left side image was shot on AUTO. Note the very shallow depth-of-field (the bush in the foreground is out of focus). Note also the overall tone of the image. It has a slight reddish cast to it. The right side image was made on MANUAL. I DECIDED that I wanted better depth-of-field as well as more overall clarity to the image. I also changed the “PICTURE STYLE” in the camera to reflect a truer sense of what the scene looked like. To my eye, it appears much more “normal” than the first one.

So, the lesson again is about making decisions about what you want your photographs to look like. Once that is done, many of the settings you need to use will become just a matter of adjustments on your camera. You can always tweak these later on in post-processing, but you will have much better results when the thought process comes first. The nature of the images you are making will also dictate which combinations of settings you will use. Trying to stop action will dictate a faster shutter speed, but may require a wider aperture which in turn, may require an increase in ISO settings. Conversely, creating a blur may require a slower shutter speed, which lets in more light necessitating a smaller aperture and maybe a smaller ISO number. This may cause a further problem because restricting light from entering the camera may require you to use a filter to keep out even more light. Any one or more of these elements will cause changes in the exposure triangle, of which you should be aware.

One thing that should be said about settings these days is that they are all made in the camera! In the days of film, aperture settings were always on the lens. Shutter speed was on the camera and of course, ISO (ASA in film days) was controlled by which film you used. It is really handy these days to have all the controls in the camera. It lets you switch lenses while maintaining the same exposure.