The Exposure Triangle

If you are new to photography, you have probably heard the phrase “exposure triangle” and I wanted to take some time to explain it, since it is foundational to understanding photography. It’s a basic topic, but an important one, so I’m going to dig in and explain the three components of the triangle (actually four!) so you hopefully come away with a solid understanding.

Let’s begin by talking about the exposure of an image. When we talk about an image’s exposure, we are talking about the amount of light that is used to capture the image. You’ll sometimes hear exposure expressed as units in EV, which stands for exposure value. Higher EV numbers are brighter exposures, and lower numbers are darker ones. Since EV is simply a measure of how much light was collected to form the image, and a brighter image collects more light than a darker one, a higher EV for a brighter image makes some sense.  

EV is usually expressed in relative terms, and is typically expressed in a unit called stops. That is, we might say that exposure A is 2 stops brighter than exposure B. A stop is just an arbitrary amount of exposure that conveniently matches some of the other parameters we’re going to explore. Expressing EV in relative terms also matches how our cameras work, as we’ll see in a moment.  

Now that we understand exposure as it relates to brightness, let’s explore how we can manipulate it with the camera. Your camera has three ways you can control exposure:

• Aperture

• Shutter speed

• ISO 

What makes these controls so interesting is that when we adjust them, they don’t just adjust the exposure. There are side effects too. It’s these side effects that give us creative control over how an image looks.

Aperture

Aperture refers to the size of the opening in the lens that lets in light. Inside your lens, there’s a small diaphragm that can open and close to vary the size of the opening. The larger the opening, the more light is let in.

Aperture is expressed in f-stops, like f/1.4 or f/16. An f-stop is the ratio between the focal length of the lens and the size of the opening in the lens. For instance, if you have a lens with a 50mm focal length and a 42mm opening, you have a lens that supports a maximum aperture of f/1.2 since 50mm/42mm is 1.2. If you adjust the opening so that it is only 6mm wide, then you have stopped the lens down to f/8, since 50/6 = 8. What confuses beginners often is that the smaller f-stops are actually larger openings and let in more light.

Like EV, apertures are sometimes expressed relative to one another, and that is usually expressed in stops. A full stop change in aperture results in the doubling or halving of the size of the opening, depending on which way you are going. For instance, going from f/1.4 to f/2 is a full stop difference, which results in the f/2 aperture being half the size of the f/1.4 aperture. It is useful to remember the full stop increments of common apertures. They are:

f/1.4   f/2   f/2.8   f/4   f/5.6   f/8   f/11   f/16   f/22   f/32 

Most cameras also allow you to specify apertures in smaller increments, typically 1/3 of a stop. For instance, you could specify f/2.2, which is 1/3 of a stop smaller than f/2.

The side effect of aperture is that the larger the opening, the narrower the depth of field, which is simply the amount of the image that is in focus. A wide open aperture like f/1.4 will have a razor thin depth of field, allowing you to capture portraits with just the eye in focus. A narrow aperture like f/16 will have nearly the entire scene in focus, which allows you to capture a landscape in full focus. Choosing the depth of field by selecting the aperture is one way you have creative control over how an image looks. 

Shutter speed

Shutter speed is simply the length of time the shutter is open when you are making an image. The longer the shutter is open, the more light is let in, and the brighter your image will be.

Shutter speed is expressed in seconds. Shutter speeds longer than a second are expressed as they are (like 2” or 10”), but speeds shorter than a second (like 1/60th or 1/160th) are usually expressed in the camera with the denominator only. That is, if you are shooting at 1/160th of a second, your camera will usually just show 160. This varies depending on the camera manufacturer though. 

Even though the values for shutter speed are intuitive, it is common to express the difference between shutter speeds in terms of stops. Doubling or halving a shutter speed results in a full stop difference. For instance, shooting at 1/100th of a second and halving it to 1/200th of a second is a full stop reduction in brightness. This makes it easy to adjust shutter speed by stops without having to memorize anything. Of course, you can also adjust shutter speed in other increments too, typically 1/3 of a stop like with aperture.

The side effect of shutter speed is that longer shutter speeds show motion, whereas shorter shutter speeds stop it. For instance, if you are taking a picture of a waterfall and choose a long shutter speed, the motion of the water will appear blurred and smooth. But, if you choose a short shutter speed, the motion of the water will be stopped, and if you have a fast enough shutter speed, you might be able to see individual water droplets.

ISO

ISO is a little more complex than the other two elements. When light hits your camera’s sensor, it creates a digital signal, which can be amplified. When the signal is amplified, it gets brighter. This means that a higher ISO value will result in a brighter image as compared to a lower ISO value.

ISO is expressed in numbers like 100, 200, 400, etc. The numbers are not arbitrary and have a specific technical meaning, but it is unimportant for digital photography. Instead, the thing to know is that 100 is the base ISO for most cameras, which results in no amplification. Values higher than 100 result in some type of amplification to the signal, which increases the brightness.

When it comes to stops, ISO behaves like shutter speed. Doubling the number doubles the amount of amplification, and halving it halves the amplification. Starting at ISO 100 and doubling the ISO, we can arrive at the full stop incremental ISOs, which are then 200, 400, 800, 1600, 3200, 6400, 12800, and so on. Of course, these can also be adjusted in smaller increments, again typically 1/3 of a stop.

The side effect of ISO is that amplifying the image boosts noise in the image. For instance, an image shot at ISO 200 will be noisier than one shot at ISO 100, although it might be difficult to tell with values that low. But, choose an ISO of 6400 or 12800 and compare it to ISO 100 and you’ll be able to see the difference.

Tying it together

Now that we understand the components of exposure, let’s understand how they relate to one another. Recall that for each component – aperture, shutter speed, and ISO – we can adjust them in stop increments. Recall also that EV is measured in stops. Putting these together, this means that if we open our aperture by one stop, we will increase the EV by one stop, resulting in a brighter image. 

Of course, you might not actually want a brighter image, which is where this starts to get fun and interesting. Let’s say you want to open your aperture to its widest amount because you are after a razor thin depth of field. If you do nothing else, your image might be too bright and overexposed, so you’ll need to compensate by changing your shutter speed to a faster speed to let in less light. Or, you could adjust your ISO to a lower value. Or, you might find some other combination where perhaps your aperture isn’t quite as wide open and your shutter speed isn’t too extreme.

The relationship between aperture, shutter speed, and ISO is called the exposure triangle. It is so named because the metaphor is that each of these values is one corner of the triangle, and that manipulating the value of one requires you to alter the values of the other in order to maintain the same exposure. If the metaphor is confusing, don’t worry about it, so long as you understand how the settings are related. 

This leads to the concept of equivalent exposures. For a given EV value, there are a nearly infinite number of settings you can choose to achieve that value. For instance, let’s say you’ve picked this exposure:

f/8, 1/100th, ISO 200

Now let’s say you’ve decided that you want a thinner depth of field, so you open the aperture by one full stop to f/5.6. To compensate, you can reduce the shutter speed:

f/5.6, 1/200th, ISO 200

Or, you could reduce the ISO: 

f/5.6, 1/100th, ISO 100 

Or, maybe you decided that f/5.6 isn’t wide enough, so you reduce it by two more full stops. To compensate, you decide to adjust the shutter speed again:

f/2.8, 1/800th, ISO 200

The beauty of this is that you can pick the combination of settings that suits the needs of the scene. You can choose to alter your depth of field, alter how long you expose for, alter how your signal is amplified, or some combination of all three.

Shooting modes

By far the most common creative choices are to choose the aperture and shutter speed; for most photographers, the ISO is a consequence of those two choices. Choosing the aperture and shutter speed are so common that nearly every camera has a dedicated dial to control each.  

In addition, most cameras have priority modes that let you choose one and have the other automatically be selected by the camera. These are named Aperture Priority (Av) and Shutter Priority (Tv). Using these modes allows you to focus on either aperture or shutter speed and let the camera worry about balancing the exposure based on your selection.

For instance, in Aperture Priority mode, you might pick f/16 because you want full depth of field in a scene, and your camera will choose the appropriate shutter speed for a good exposure. Many cameras also allow you to control whether you select the ISO, or the camera does. Even in these automatic modes, you still have control, since you can typically specify settings like the maximum automatic ISO value you want the camera to choose or the slowest shutter speed the camera should pick. Some cameras even have a safety shift feature that automatically overrides those settings and picks a value outside of them in extreme situations.

There are other priority modes you can experiment with, depending on your camera. Many new Canon cameras offer a new mode called Flexible Priority (Fv), which allows you to easily delegate automatic control of the whole exposure triangle to the camera, but quickly take back control of any component when you need it. Of course, all cameras have a Manual mode (M) where you choose all three elements of the exposure triangle yourself.  

Metering and exposure compensation

You might be wondering how the camera chooses the correct exposure for a given scene. That is, how does it know the scene is too bright or too dark?

Your camera has a computer on board that is designed to meter the scene and decide how bright of an exposure you need in order to properly expose an image. This computer is activated whenever you start metering, which is usually when you half-press the shutter button. You can control how the metering computer works by adjusting the camera’s metering mode. Most cameras offer a variety of modes, but usually include settings to meter the whole scene or just the center of the image.

The manufacturer of the camera has pre-programmed what should be a reasonable exposure, which is expressed to you as the zeroed exposure. Once activated, the metering computer measures the amount of light in the scene to decide how far away from the zeroed exposure you are. It then shows you a graph in the viewfinder that indicates how far (in stops) over or under the zeroed exposure you are. If you are using an automatic mode like Av or Tv, it will adjust the settings you are not controlling to make the exposure the zeroed exposure.   

That’s all well and good, but what if you don’t like the zeroed exposure? What if the camera made a mistake and the image is too bright? Or, what if you are going for a high key image where some overexposure is necessary for the look, or a low-key image where you want the final product to be intentionally dark?  

This is where exposure compensation comes in, which is the fourth component of the exposure triangle. Exposure compensation allows you to alter the zeroed exposure and force the camera to accept a brighter or darker exposure then it otherwise would. Naturally, exposure compensation is expressed in stops, so you can choose a value 1 stop over or 2 stops under the zeroed exposure for a different look. 

Since it is so common to adjust, exposure compensation is usually given a dedicated dial or control on the camera. In fact, in Aperture or Shutter Priority modes, the primary deal controls either the aperture or the shutter speed, and the secondary dial usually controls the exposure compensation. In these modes, manipulating exposure compensation has a side effect of manipulating shutter speed (and ISO if you are in automatic ISO mode). 

Wrapping up

Hopefully you now have a better understanding of the exposure triangle. My advice if you are just starting out is to grab your camera and experiment with the triangle. Pick an exposure, and then pick an equivalent one, and see how the image changes. Experiment with exposure compensation. Try out manual mode where you control all of the variables and see if you can get a good exposure. Learn the ins and outs of how your camera manipulates the exposure triangle until it is second nature to you.

If you do that enough, you will develop an intuitive understanding of exposure, and will then be able to shoot confidently in any photographic situation you might encounter. Have fun!