The Truth About the Exposure Triangle, and Should You Use It?

Making the relation between aperture, shutter speed, and ISO visible is the idea behind the exposure triangle. I have seen many beautiful drawings of the exposure triangle, but what does this diagram really show? Let’s find out.

When writing my Dutch e-book “Licht Vangen,” which translates into Catching Light, I needed to cover the exposure triangle. Although I’m photographing for over four decades, I never used this diagram. But I have seen the many different versions on the internet, and inside popular photography magazines. These drawings often look amazing, covering more than exposure alone. Just do a search on Google, and see for yourself.

Many photography courses cover the exposure triangle, emphasizing its importance in understanding the relation between aperture, shutter speed, and ISO. I even hear about photographers who claim to use the exposure triangle exclusively, to keep a perfect control over the exposure for every photo they take.

The Exposure Triangle in Detail

First, let’s have a closer look at the exposure triangle. If you know what is, you might want to skip this chapter. But perhaps it’s good to keep on reading, to have everything clear.

To understand the exposure triangle you must know about the settings we can use to get the exposure correct. Aperture is the first one, controlling the amount of light that goes through the lens. Shutter speed is the second one, controlling the time light is hitting the sensor. The ISO is the third one, and although it doesn’t control the amount of light in any way, it resembles the sensitivity of the analog film. I don’t want to go into depth about ISO, and keep it simple by assuming it is sensitivity.

These three setting need to be in balance in order to have the right exposure. By placing the three settings into a triangle, we can visualize the relation. Sometimes we see the aperture, shutter speed, and ISO at the corners, as if these settings are puzzle pieces that need to fit. More often we see these on the sides of the triangle, together with the possible values of each one. By drawing a line between each setting we can visualize a correct exposure in the triangle.

What the Exposure Triangle Doesn’t Show

By this simple description you might already see what wrong with this picture. You can draw random lines and end up with a every possible combination. It clearly doesn’t tell anything about a correct exposure.

To draw the correct lines inside the exposure triangle, you need to have a light meter that can measure the amount of light available. For that you need choose two settings, of course, and the light meter will measure the third one. Often we see how the ISO is kept as low as possible, and the aperture is used for a desired depth of field. In other words, you have the sensitivity of the sensor set, together with the amount of light that passes through the lens. With these two settings, you will need a light meter to know how much time is needed before the exact amount of light has hit the sensor.

Let’s go back to those photographers that use the exposure triangle exclusively to keep control over the exposure. By using the exposure triangle alone, they won’t know the exact settings. They must use a light meter also.

What the Exposure Triangle Does Show

You need to choose two out of three settings, and measure the amount of light to know the third setting. Only then you can have a correct exposure. Since every modern camera has a light meter built in, you can set the ISO, and the aperture, just like in my example, and measure the corresponding shutter speed. For that you don’t need a exposure triangle.

But what if you end up with a shutter speed that is not to your liking. Perhaps it is too slow, but making it faster will lead to an under exposed image. After all, in that case the shutter isn’t open long enough to gather enough light for the correct exposure. In that case you need to change the aperture or the ISO to compensate the faster shutter speed. For that you can use the exposure triangle.

If you have determined a correct exposure, and you want to change on of the three settings, the exposure triangle can help you make the right correction. That is why a good exposure triangle mentions if you get with more light, or less light when changing a setting. This way you can read easily see in which direction you need to change a setting.

What More Can the Exposure Triangle Show?

Using an exposure triangle as mentioned in the previous chapter will help you understand the consequence of the compensation you might perform. Changing the aperture, shutter speed, and ISO does not only change the amount of light for the exposure. It also has an effect on the image.

As you know, the aperture is used to control the depth of field. A smaller aperture will increase the depth of field, while a larger aperture will decrease the depth of field. Shutter speed affects the motion of a subject. If shutter speed becomes longer, motion will become visible. And increasing the ISO, which is basically the signal amplification of the sensor, will lead to a higher noise level.

Most popular exposure triangles show these effects in some way or another. This way you will know what effect the exposure compensation will have on the image. If you compensate a faster shutter speed with the larger aperture, it will lead into a smaller depth of field. On the other hand, if you use the ISO to compensate it might lead into a higher noise level.

In this case, the exposure triangle isn’t showing the exposure, but the effect of depth of field, motion, and noise when changing settings. Be aware, the exposure triangle won’t show you the exact depth of field, motion blur, or noise levels. It is just telling you if the change will increase, or decrease these effects. The exposure triangle tells the relative effects, not the absolute ones.

Is It Wise to Use the Exposure Triangle?

The answer to this question can only be answered by you. I think it is possible to use the exposure triangle in a good way, but not for determining the exposure itself. If you have determined a correct exposure, the triangle can be used to see how you must compensate a change in setting, and what the effect on depth of field, motion blur, or noise levels will be on the end result.

For me, the exposure triangle has no use. During my four decades of photography I have gained enough knowledge to know how to compensate a exposure setting, and how it will affect the image. But for the beginning photographer, or the one that has difficulties imagining these things, the exposure triangle can be very helpful. Just make sure you use one that mentions the effects of aperture, shutter speed, and ISO settings, both for exposure as well as depth of field, motion blur, and noise levels.

Have you used the exposure triangle, or do you still use it today? Please share your experience with this little graph, and how you use it in the comments below.


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