The exposure triangle is one of the most fundamental concepts any photographer needs to master to take control of their images. If you struggle to understand how it all comes together to produce a complete exposure, this article is for you.
The exposure triangle refers to the three parameters that control exposure: shutter speed, aperture, and ISO. Together, these three settings combine to produce a complete exposure; thus, it is crucial that any photographer have a mastery of both the theory behind them and their practical application. However, these settings can be a bit abstract and not particularly intuitive. So, let us discuss an analogy of how your camera interacts with light to make these concepts a bit easier to work with.
The Pipe, the Water, and the Bucket
Imagine light flowing to your camera is like water flowing through a pipe into a bucket. The photons of light are the water molecules. We can turn the flow of water on or off with a spigot at the end of the pipe. We can also choose the size of the bucket we use. We can also choose the diameter of the pipe.
Our goal is to completely fill the bucket with as pure water as possible without it overflowing (overexposing). Leaving too much of the bucket empty would be underexposing. Unfortunately, our bucket is not quite clean; it has some dirt residue on the sides that will contaminate the water once it is in the bucket.
There a number of variables in this scenario that correspond to various photographic parameters, so let’s review them. We have the spigot that turns the flow of water into the bucket on or off. We can change the diameter of the pipe. We can choose the size of the bucket we use. What we can’t control is the fact there is some dirt on the inside of the bucket or the pressure of water in the pipe.
Pressure: Available Light
We do not control over the amount of pressure, just like we do not get control over the amount of available light. If there is a lot of pressure in the pipe, there are a lot of water molecules flowing through it. This would correspond to a situation with a high amount of available light. If there is low pressure in the pipe, not a lot of water flows through it, which corresponds to a low amount of available light. You could conceivably change the pressure in this scenario by adding artificial light. You could also reduce the pressure with an ND filter, but for the purposes of our discussion, we will assume you have not done either of those things.
The Spigot: Shutter Speed
The spigot corresponds to shutter speed. As long as it is open, water (light) is flowing into the bucket. If the water pressure is high in our pipe (lots of available light), we will not have to leave the spigot open for very long to fill our bucket (short shutter speed). On the other hand, if the water pressure is low, we need to leave the spigot open for a longer time (longer shutter speed), as the flow of water (accumulation of light) will be slower.
The Pipe Diameter: Aperture
The diameter of the pipe corresponds to aperture. The wider it is, the more water (light) can flow through the pipe at any given time. If it is narrow, flow is constricted, and less water (light) can make it to the bucket (sensor). If there is a lot of pressure in the pipe (high available light) or we leave the spigot open a long time (long shutter speed), we can get away with using a narrower pipe (narrow aperture). On the other hand, if there is not a lot of pressure in the pipe (low available light) or we do not leave the spigot open very long (short shutter speed), we will need a wider pipe to fill the bucket (wider aperture), or we need to use a smaller bucket (see below).
The Size of the Bucket: ISO
The size of the bucket we use corresponds to ISO. The smaller the bucket (higher ISO) is, the less water (light) it will take to fill it. A larger bucket (lower ISO) will take a longer time to fill. If we have some combination of high pressure (lots of available light), a wide pipe (wide aperture), or leaving the spigot open for a reasonably long time (longer shutter speed), we can afford to use a big bucket. On the other hand, if the pipe does not have a lot of pressure (low available light), it is narrow (narrow aperture), or we can’t leave the spigot open very long (short shutter speed), we will not be able to fill a very big bucket, so we need to resort to using a smaller bucket (high ISO).
The Dirt in the Bucket: Noise
After reading the above paragraph about the size of the bucket (ISO), you might be wondering why we would ever use a large bucket (low ISO). Remember that our buckets are imperfect, as they have a bit of dirt residue stuck on the inside. If we use a small bucket (high ISO), there will not be much water to dilute the dirt, and the water will be relatively dirty. This dirty water corresponds to a noisy image. On the other hand, if we use a relatively big bucket (low ISO), the small amount of dirt on the sides of the bucket will not be very noticeable, as it will be diluted by the water. In technical terms, this is referred to as a high signal-to-noise ratio.
Remember, our goal was not only to fill the bucket, but to do so with the purest water possible, meaning we want to use as big a bucket as possible to dilute the dirt (noise) as much as possible. This corresponds to the cleanest water (the lowest noise/cleanest image) possible.
Understanding how the three exposure parameters interact is crucial to being able to create properly balanced exposures. What is important to note is that this discussion only talked about how to create a proper exposure from a technical perspective. Each of these parameters also give you creative tradeoffs depending on their settings. For more on those, check out this article.