The exposure triangle claims to explain the relationship between shutter speed, aperture, and ISO. At first glance, it looks like a useful diagram, until you realize that it’s not all what it's cracked up to be.
It's Pretty, but Not Accurate
When you first see the Exposure Triangle it’s an attractive graphical chart or diagram showing the range of each setting and the effect each setting has upon the exposure. That’s where the usefulness ends. Then newcomers start asking about where the current exposure is indicated in the triangle, only to find out it's not.
WClarke, Wikimedia, CC BY-SA 4.0
The triangle indicates that if you go up in ISO you brighten the image, but as you do you get closer to the corner that also indicates more depth of field and a darker image. This is where it all falls apart.
Placing the elements of exposure in a graphical representation of a triangle implies that there is a relationship between each side and/or the corners. That's the whole purpose of a diagram like this, to depict the relationship between items. The Exposure Triangle does nothing to explain the relationship between these items. It simply takes three things and puts them in a triangle.
The current exposure is not represented in the triangle. When one setting changes it doesn’t explain how you can change either of the other two settings to maintain the proper exposure.
I'm very technical; I'm a software developer. I've written code to make charts and graphs to graphically represent data. When I first saw the Exposure Triangle I stared at it for a little while trying to figure out how the sides interacted with each other. After a while of analyzing it, I realized that they were not related in any way and it was simply three settings placed in a triangle for no other reason than a triangle has three sides.
A Better Diagram
I'm not saying that this is the best that it gets, but I think the following image does a little better job at describing what will happen when you change a setting:
In fact, it’s easier to explain that for a given exposure, using the above chart, that if you go darker on one setting you can simply go brighter on another setting the same number of stops to maintain that exposure. This is because each stop either lets in (exposes for) half as much or twice as much light as the previous stop. That’s it. Half as much, twice as much. As for ISO, it doesn't let in more or less light, but it does allow for the changing of shutter speed and aperture, which does.
ISO Isn't Part of Exposure
Exposure is the amount of light falling per unit area on the sensor. Technically ISO isn't a component of the exposure. It's simply amplifying the sensor values and modifying the captured image so that it will appear the "same" as it would have if the image had been properly exposed at ISO 100 (or whatever the base ISO of the sensor is). It's similar to the volume on a radio, the incoming signal doesn't get any stronger, it's just being played louder (amplified), static and all. But since sensitivity is simulated on digital cameras (as apposed to actual sensitivity of film), we'll pretend it's part of the exposure since it's what we work with when taking a photo.
The Myth Will Live On
I don’t think the Exposure Triangle is ever going away. It’s like the myth that swimming after eating will give you cramps. It’s been debunked a million times, yet the myth still lives on. In fact, my grandkids just repeated it to me the other day.
It’s only THREE THINGS. Each only has one primary attribute. Whatever happened to “It’s easy as 1-2-3”? So what should we call it? The Exposure Triad? The Three Pillars of Exposure? The Exposure Trinity?
How about just the three primary settings of exposure? What is your opinion of the Exposure Triangle?