The exposure triangle is the most crucial concept to understand in photography. It is what governs how light or dark your image will appear. Even if everything else in the photograph is on point, if it is too dark or too light, it will be difficult, if not impossible, to recover.
I am willing to bet that every photographer, at one point, has underexposed or overexposed a photograph. Whether we neglected to consult the light meter, didn’t know how to correct it, or didn’t even know what it was, we’ve all been there. I am here to help you understand what the exposure triangle is and how it can help you achieve a correctly exposed photograph every time!
The exposure triangle makes up the fundamentals of photography on which everything else is built. I’ve said before, in the post studying Ansel Adams, that an adverse side effect of modern digital photography is the extraordinary number of functions that cameras are now capable of. Principally, though, they are all hinged around one thing: the exposure.
Here I will cover:
1) what the exposure triangle is
2) what the three variables involved are
3) how to use them to achieve perfect exposure
4) and why you need to know how to use them
This post is for you if you own a digital camera with some degree of manual settings (or one who is considering purchasing some such camera) and want to learn how to wean yourself off auto mode. Alternatively, if you are someone who has tried manual settings but couldn’t quite figure them out, this post is also for you. Whereas, if you’re happy shooting in auto mode and aren’t noticing any limitations or drawbacks, you may not find this post worth the read.
Let’s take a closer look at the base of all photographs: the exposure triangle.
First off, what is exposure?
Exposure in photography refers to the amount of light that reaches the camera sensor.
Then, what is the Exposure Triangle?
The exposure triangle is commonly referred to as the relationship between the three variables that determine the exposure of a photograph: aperture, shutter speed, and ISO.
Finally, what does that mean for us?
Each frame has ‘optimal exposure.’ Like a mathematical equation, there is a correct answer to where the photograph is neither too dark nor too light. This bed feels just right! (Goldilocks reference, anyone?)
Ultimately, the goal is to balance all three exposure triangle variables to achieve the correct exposure.
It can be tricky to understand how each variable works and even more problematic to know how to put them into practice. We will get deeper into that. For now, let’s talk about what the variables are and their relevancy to the exposure triangle. Hang tight; there’s one more thing we need to cover.
First and foremost, A Stop of Light
A stop of light in photography refers to the doubling or halving the amount of light in the exposure. Understanding what a stop of light is is the key to understanding how to effectively use the variables of the exposure triangle. Stay with me.
As stated above, each photograph has an optimal exposure; that is, there is a correct amount of light required to expose any given frame correctly. Photographers can add a stop of light by doubling the amount of light entering the camera, thereby doubling the exposure. Conversely, they can take away a stop of light by halving the amount of light entering the camera, thereby halving the exposure.
Common phrases among photographers when referring to adding or taking away stops of light is stopping up or stopping down, respectively. I will refer to this phrasing again.
So, how do we add or take away stops of light, you ask?
By adjusting the variables in the exposure triangle.
On to the good stuff.
Shutter Speed
Shutter speed refers to the length of time that light is hitting the sensor. In other words, the length of time that the shutter is open. Put yet another way, the speed at which the shutter opens and closes. Hence, the shutter speed.
This is the most obvious point on the exposure triangle to comprehend. Measured in seconds (more commonly as fractions of a second), as you double or halve the length of time the shutter is open, you are adding or taking away a stop of light.
Side Note:
More practically speaking, rather than quantifying the length of time the shutter is open, I prefer to think in terms of the speed at which the shutter opens and closes. It’s a distinction that warrants consideration because it flips things around.
For instance, instead of decreasing the length of time the shutter is open to decrease exposure, I will increase the speed at which the shutter opens and closes to decrease exposure. The result is identical. Yet, I find it easier to think this way because of how the camera controls function.
Shutter speed is expressed in fractions of a second, but some cameras (mine included) show you only the second number in the fraction (as pictured below). So, continuing with my example, as you twist the dial to the right, the number increases as does the shutter speed. To think about the length of time the shutter is open, you create a three-part sequence:
1) apparent increasing shutter speed translates to
2) a decreasing length of time the shutter is open, which means
3) a decreasing exposure
Alternatively, thinking with shutter speed eliminates number 2:
1) apparent increasing shutter speed translates to
3) a decreasing exposure
All you have to remember is that increasing the shutter speed decreases the exposure, which you have to remember anyway.
Back to the Point:
However you choose to think of it, stopping up (adding a stop of light) here means a slower shutter speed or more time the shutter is open. Stopping down (subtracting a stop of light) means a faster shutter speed or less time the shutter is open.
These increments are full stops of light, doubling or halving the exposure.
You might have noticed that your camera offers more shutter speed options in between these increments (1/10s, 1/200s, 1/640, etc.). As you could guess, these adjust the exposure by part stops of light. For the sake of simplifying, I will continue to illustrate using only full stops, which are what needs memorizing. However, know that there is wiggle room between the stops to fine-tune the exposure with greater precision. The same will be valid for aperture and ISO.
Aperture
Aperture is the size of the window that light is allowed through. The larger the diameter, the more light enters.
Perhaps the most confusing of the variables, setting the aperture doesn’t appear logical on the surface. Aperture is set using f-stops. Without getting technical, the fact is that the larger the f-stop, the smaller the aperture. Consequently, a large f-stop number means less light. Uhum, what?
This is the sequence for full stops of light, doubling or halving the exposure.
The numbers move by a division of the square root of two (rounded to the nearest single decimal place) because the area of a circle scales with the square of its radius. The result is half the size or double the size of its neighbours, per a stop of light.
Just like shutter speed, cameras have additional f-stops available that adjust by part stops of light. These offer the same wiggle room as shutter speed, but the increments of one full stop of light (charted above) need memorizing.
TO EMPHASIZE, THIS IS ALL THAT ANY PHOTOGRAPHER NEEDS TO KNOW ABOUT APERTURE CONCERNING THE EXPOSURE TRIANGLE:
1) the sequence of f-stops for one full stop of light (pictured above)
2) the fact that the larger the f-stop, the smaller the aperture, the less exposure, and vice versa
The camera and lens handle the rest.
If you’re curious to learn how the math makes sense, keep reading. Otherwise, skip on to ISO.
Bonus Material: The Math
F-stop is short for focal-stop. Focal refers to the focal length of the lens, and stop refers to the stop of light. As mentioned before, we use f-stops to set the aperture, but we (the camera, really) also need to consider the focal length of the lens. The equation looks like this:
F-stop = focal length / diameter
Because we control the f-stop and not the diameter (the camera calculates that for us), we will rearrange the equation like this:
Diameter = focal length / f-stop
When you do the math, you’ll see that a small f-stop results in a large diameter, and vice versa. Let’s take an example.
50mm focal length / f-stop of 8 = 6.25mm diameter
Smaller aperture equals larger diameter:
50mm / f4 = 12.5mm diameter
Larger aperture equals smaller diameter:
50mm/f16 = 3.125mm diameter
Again, this math is not necessary for the practical use of aperture. The optics in the lens do it all for you. However, if you were curious, there you have it.
Back to the Point:
To summarize, the aperture is the size of the shutter when it opens. We use f-stops to set the aperture. The smaller the f-stop, the larger the aperture, the higher the exposure. The larger the f-stop, the smaller the aperture, the lower the exposure. Using the chart above, stopping up means setting a lower f-stop to achieve a larger aperture, increasing exposure. Stopping down means setting a higher f-stop to get a smaller aperture, decreasing exposure.
But wait, there’s more!
ISO
In simple terms, the ISO is the camera sensor’s sensitivity to light. It is expressed as a number ranging from around 100 or less to about 12 000 or more. The exact ISO range depends on the camera.
Like shutter speed, doubling the number will add a stop of light where halving will subtract one, thus adjusting the exposure.
This is the scale for full stops of light, doubling or halving the exposure.
Moreover, just like shutter speed and aperture, additional ISO stops exist in between that provide part stops, again giving you that wiggle room. The ones pictured above are what need memorizing.
Put in more practical terms, a lower ISO number is appropriate when there is a lot of light available; the sensor doesn’t have to be particularly sensitive to it. On the other hand, we need a higher ISO number when light is hard to come by; the sensor needs to be ultra-sensitive to capture the little that’s there.
Sadly, raising the ISO has consequences in the way of image quality. The most noticeable side effect would be grain or noise. Additionally, the colour vibrancy and dynamic range will be increasingly lacklustre the higher ISO you go. In plain language, the camera’s ability to render details is hindered when it’s busy just trying to find the little amount of light. For these reasons, we recommend keeping the ISO as low as possible to maximize image quality.
The Subtle Art of Balance
As I said at the beginning of this post, the exposure triangle’s ultimate rule is balance. Therefore, if you stop down one variable, you need to stop up another, and vice versa, to maintain exposure. Sounds simple, right?
The Twist
Of the three variables, shutter speed and aperture affect additional creative elements of the photograph beyond exposure. As you might expect, there are circumstances in which you may want or need an extra fast shutter speed or an unusually small aperture, for example. To accommodate this, you will have to balance the exposure by adjusting one or both of the other variables accordingly.
How do you know if you have the correct exposure?
Digital cameras have a built-in light meter that measures the exposure in your frame for you.
The numbers on either side of the 0 represent full stops of light.
As mentioned earlier, many cameras will offer part stops of light falling in between these full stop increments, so the dots represent those part stops.
To familiarize yourself with this function, hold the camera to the same frame (or mount it on a tripod) and experiment in manual mode. Adjust the shutter speed, aperture, and ISO and see what happens to the light meter. Try it in bright sunlight, shade, and/or indoors to see the varying effects.
Stay tuned for a more detailed post on identifying and correcting overexposure and underexposure.
Camera Modes
To help get you started taking photos without auto mode, digital cameras generally offer at least three other modes:
1) shutter speed priority – you control the shutter speed; the camera adjusts the aperture for optimal exposure
2) aperture priority – you control the aperture; the camera adjusts the shutter speed for optimal exposure
3) manual – you control both; the camera does nothing
As you can see, there are additional shooting modes. P is for program, and the rest are variations of Automatic mode. Let me know if you’re interested in a full description of those modes. For this post, I have focused on the semi-manual and manual modes.
Additionally, you can have the camera auto select your ISO. The camera will adjust the ISO only if it cannot reach optimal exposure using the specified or calculated shutter speed and aperture. If you do not have this setting turned on, the priority modes listed above will not affect the ISO. In other words, you will have to adjust the ISO yourself, if necessary.
These shooting modes and settings are designed to help balance the exposure for you, which can be ideal for some circumstances, even for experienced photographers. In addition to these modes, there are many other settings and features many digital cameras have that relate to exposure. We’ll get into that another time.
Summary
Shutter Speed, Aperture, and ISO are the variables that make up the exposure triangle. They work together to control how much light hits the camera’s sensor – the exposure.
A stop of light – the doubling or halving of the exposure – is the measurement used between all three variables. You decrease the exposure by stopping down (subtracting a stop of light) in one of the variables, and you increase the exposure by stopping up (adding a stop of light) in one of the variables.
Balance is key. When you change one variable, for whatever reason, you must adjust another to maintain exposure. On the other hand, if the exposure is already incorrect, you consult the exposure triangle to change one or more of the variables to correct it.
A camera’s light meter shows you the frame’s current exposure level. It works in real-time to tell you if you’re correctly exposed, underexposed, or overexposed, and by how many stops.
Finally, digital cameras offer shooting modes and settings that help you balance your exposure. These are great tools to get started experimenting with the exposure triangle.
Conclusion
For now, I hope this guide was comprehensive enough to be helpful. I hope it inspired you to dabble in manual modes and settings a bit more.
As a final note, a lot of what I’ve learned has been through trial and error. Never be afraid to take the time, and the shots, to experience these things first hand. Play around with your camera, switch up the settings, and see what happens.
My goal is always to help you become a better photographer and to become a better photographer myself.
Let’s give it a shot.