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Why the Exposure Triangle is Completely Useless

Understanding Exposure: Why the Exposure Triangle method is complete BS

The "exposure triangle" is a common tool for teaching beginners about exposure. But here's the truth about the exposure triangle: it's a terrible learning tool that is more likely to harm than help beginner photographers. I believe this tool is adopted by teachers who actually aren't that good at teaching a simple concept. They can't communicate shutter speed, aperture, ISO, and exposure in an effective way, so they teach via this "learning trick."

The Problem with The Exposure Triangle

Take it from someone who teaches photography for a living: the exposure triangle makes the simple concept of exposure seem much more complicated than it is. I know exposure, shutter speed, aperture, and ISO better than I know myself and I can barely make sense of the exposure triangle. I'm serious. It's comical how unnecessarily complicated it is. It's like watching one of those Rube Goldberg machines that turns the simple task of toasting bread into a thousand-step process.

But aside from the fact that it's overly complicated, the inherent problem with the exposure triangle is that it relies on memorization rather than understanding. You are expected to visualize a diagram with shutter speed, aperture, and ISO labeled at each respective corner, trying to remember which corner means more light, which one means less light, which side deals with motion blur, which side deals with depth of field... Unless you have a photographic memory, you're going to have to carry along an exposure triangle cheat sheet for reference.

So then what's wrong with carrying around a cheat sheet? Well, taking pictures with the help of a cheat sheet is like trying to ask a girl out on a date using a pre-written script - it'll work until she says something you weren't prepared for. The "cheat sheet" method of shooting is too slow and too inflexible. You have to be able to think fast, think on your feet, and adapt to situations quickly. An exposure triangle cheat sheet can't do that.

The Better Way to Think About Exposure

The way I teach my students about exposure doesn't rely on memory tricks like the exposure triangle because learning by memorization doesn't work. Memories can't be trusted. But if you understand how something works, then you'll almost never forget it and the whole process becomes easy. So let's talk about how to understand exposure so that we no longer need to rely on memory.

For now, let's ignore all the numbers connected to shutter speed, aperture, and ISO. The numbers are secondary, first you must understand the mechanics of exposure. If you think about the mechanics of how an exposure is made, then you'll be able to problem solve any exposure situation - no need for an exposure triangle cheat sheet. The numbers will follow.

How Exposure Works

Creating an exposure is simply the process of recording light on a photosensitive material. Traditionally that light was recorded on film, now digital cameras record light with a photo-sensitive computer chip called the "image sensor." Getting a "correct exposure" means that you recorded the correct amount of light - not too much, not too little. If you didn't record enough light, the picture would be "underexposed" or "too dark." If you recorded too much light, the picture would be "overexposed" or "too bright."

Light is made up of particles just like anything else. It's almost like water, except we can't touch it or feel it. Getting the right amount of light to your sensor is a little like getting the right amount of water to a sponge. So let's forget light for a minute. Let's talk about water.

Let's say I have a big sponge that I want to saturate with exactly 20 ounces of water from a garden hose. I can't have too much or too little - it needs to have just the right amount of water in it. And let's say I have to turn on my garden hose for exactly 1 second to get my 20 ounces. Simple enough: Garden hose turned on for 1 second and I get my 20 ounces.

Well what's going to happen if I switch out my garden hose for a fire hose? Will I have to leave my faucet on for longer or shorter now that I have a bigger hose?

Shorter. Duh. It's a bigger hose, so more water comes out, so I won't need to turn it on so long. Now how did you figure this out? Did you refer to your "water, hose, faucet triangle" on your "water triangle cheat sheet"? No, you just thought about it. This is how you should think with light and exposure. Don't rely on an exposure triangle, just think about the flow of light to your image sensor. The sensor in the back of your camera - the thing that creates the image - works just like a sponge, but instead of soaking up water, it soaks up light.

How exposure works: DSLR Camera mirror

Above is an image looking into the camera with the lens off. This is the camera "at rest" meaning it's not taking a picture right now. You'll notice there's a mirror there. That allows you to see what the lens sees before taking the picture. 

Below is an image looking into the camera while it's taking a picture. The green box in the back is the image sensor. That's what collects light (like a sponge) and turns it into an image. The mirror has to move up out of the way so the sensor can soak up the light. When the photo is done, the mirror will go back down. 

How exposure works: the digital image sensor

The aperture inside the lens is simply an adjustable opening in the lens that allows you to let more or less light through. This would be like the size of the hose. Which one of the following aperture openings would let in more light? Of course, the larger opening will.

Above is a wide aperture (low f-number), below is a small aperture (high f-number).
Guess which one will let more light through...

The shutter is the mechanism that "opens up the flow" of light to the sensor. It's like turning on the faucet. Which of the following shutter speeds (that's the duration of time the shutter is open) will let more light through: 1/100 of a second or 1/2 of a second? Well, 1/100 of a second is a much shorter time period than 1/2 of a second, so 1/2 of a second will let more light through.

How exposure works: the shutter

Here is a view of the shutter with the mirror held up out of the way. You'll notice the shutter is a set of overlapping blades that blocks the sensor from light. It keeps in the sensor in complete darkness until you actually take a photo. When you do, the shutter snaps up out of the way like a window shade so that light can soak into the sensor. When the picture is done, the shutter goes back down to block off the light. 

When you make an exposure, the shutter opens up, light flows through the aperture opening, soaks into the image sensor like water soaking into a sponge, then the shutter closes off the flow of light once enough light has been recorded.

Shutter opens > light flows through aperture > image sensor soaks up light > shutter closes. Faucet opens > water flows through hose > sponge soaks up water > faucet closes. It really is that simple. Here's a diagram demonstrating the concept:

Understanding Exposure: How exposure works on a DSLR camera

In the diagram above, you can see the aperture, shutter, and sensor all indicated along with how their setting is measured (the aperture is measured by the f-stop, the shutter is measured by the shutter speed, and the sensor is measured by the ISO). 

In the diagram below, the labels are gone to remove the clutter. This is how your camera looks when it's not taking a picture - when it's "at rest":

Understanding Exposure: How exposure works on a DSLR camera

When you take a picture, all these mechanisms have to work together to get light to the sensor so it can soak up the light and create an image. It goes like this: The mirror moves up out of the way, the aperture closes down to whatever you set it to, the shutter opens up, and the sensor soaks up the light to create an image, like this...

Understanding Exposure: How exposure works on a DSLR camera

...above is a diagram of the camera as it looks while it's taking a picture.

Some time later - maybe 1 second later, maybe 1/8000 of a second later depending on the chosen shutter speed - the camera can cease collecting light. The shutter closes to shut off the light, the aperture opens back up, and the mirror goes back down. Like this: it's back to it's "resting position" ready to take another photo.

So again, shutter opens > light flows through aperture > image sensor soaks up light > shutter closes. No need to make it more complicated than that.


Changing One Setting Affects the Other

Much of what the exposure triangle is trying to illustrate is that if you adjust one setting (shutter, aperture, or ISO), then another setting will have to change, too, assuming you want to keep the exposure the same (i.e. collect the same amount of light). But again, you don't need an exposure triangle to understand this. It's as simple as trying to get water into a sponge. If I use a wider hose to feed water into that sponge, I won't have to leave the hose on so long. If I use a narrower hose, I'll need to leave the flow on longer. Simple.

It's no trickier than this with exposure in photography. If I open up my aperture wider, then I won't need to leave the shutter open so long (the shutter speed will be faster). If I close the aperture down to a smaller opening, I'll need to leave the shutter open longer to make up for the loss of light. More on the aperture, less on the shutter. Less on the shutter, more on the aperture. You get the idea.

But what about the ISO? Well, let's say I had some way of making my sponge (the image sensor) much better at soaking up light. In other words, I was able to increase its absorbency. Well, if I have a more absorbent sponge in my water analogy, I wouldn't need to leave the faucet on so long since it's better at soaking up water. Same goes for light. If I can make my image sensor more absorbent to light, it won't need so much time to soak it up. This is what raising the ISO does. When you increase the ISO number, you're making the image sensor more absorbent to light. As a result, the shutter will only need to "turn the flow on" for a brief time. This is why raising the ISO results in faster shutter speeds.

The Bottom Line

The exposure triangle doesn't work. It's a terrible learning tool. There is no shortcut to learning the shutter speed, aperture, and ISO. Exposure is a concept that must be understood, not memorized. Cheat sheets and the exposure triangle won't get you there. Of course you won't be an expert from this short 1,000-word explanation either. My goal here is only to show you that there is a way to understand these things through logic, analogies, and simple physics without the need for a confusing exposure triangle that relies on memorization. There is much more to it that my short summary here. This is just a foundation. That's why more than half my Introduction to DSLR Photography online course is devoted to these things alone.

Sorry to throw a sales pitch at ya, but if you want to learn these things fully, check out my Introduction to DSLR Photography online course here (perfect for beginner photographers) and in my How to Shoot in Full Manual online course here (for intermediate to advanced shooters). With a clear, concise explanation aided by example images, diagrams, and videos, you can learn exposure better than any exposure triangle can deliver.

Please download the free lesson guide and video with the full explanation of shutter speed here. Download the free lesson guide and video fully explaining the aperture here.