Learning the exposure triangle – the 3 main settings of ISO, aperture, and shutter speed – will put you on the path toward creating great photographs. For new photographers, the exposure triangle creates a framework for how and when to make adjustments to get the right exposure.
There will be a specific exposure that most people will consider to be “correct”. Your camera’s built-in metering system is always trying to get you perfectly between all black and all white – a nice “middle zone”. It will measure the brightness of your scene (or certain areas of your scene depending on your metering mode) and tell you if your image is going to end up too dark, too bright, or just right. Your ISO, aperture, and shutter speed all affect exposure.
Depending on the image, you might want to underexpose or overexpose your image in order to create a specific mood. To do this easily, you’ll want to full understand what ISO, shutter, and aperture are and how the behave together.
What Is the Exposure Triangle?
The exposure triangle demonstrates that there are three exposure variables that all affect each other. They don’t work entirely independently. Changing 1 thing affects the other 2. Once you know how it works, though, you can start manipulating your exposures for unique results instead of relying on your camera’s auto mode to provide you with even, boring, “correct” shots. Also, the camera simply isn’t always good at being “correct” in auto mode. It sometimes needs a human touch. Let’s go through each side of the triangle one-by-one.
For those short on time, all you need to know is big ISO number = more light/brighter exposure and small ISO number = less light/darker exposure. Read on for details.
ISO is the measurement of a film’s sensitivity to light. Some of you may remember it as being referred to as a film’s “speed” before the DSLR era took hold of the term. Now it is the expression of a camera sensor’s “signal gain”– its ability to input light and output an image. Before, you’d use different film speeds for your various lighting conditions. We don’t switch out sensors to use more or less sensitive ones – we’re stuck with just the 1 our camera comes with. Instead of swapping film, you can dial your ISO up or down and change the amount of amplification applied to the sensor’s reading. Some cameras allow you to amplify a lot and people find this really desirable, which is why you always hear people go on about whether a camera is “good in low light”.
Raising and lowering your ISO number will change your sensor’s sensitivity to light but it will also change your sensor’s sensitivity to undesirable grain and artifacts. The higher the ISO number, the more grain and artifacts you will find in your image. As camera sensors become better and better, these artifacts will become less of an issue. Until then, grain is something to keep in mind when you want to increase brightness in your scene using ISO adjustment alone.
When we talk about ISO nowadays, we’re commenting on a camera’s sensor but with aperture, we’re talking about its lens. If lenses are like an eye then the aperture is its iris. There are physical blades inside of lenses that form a circle that get wider or tighter like a pupil.
Still mostly skimming? What you need to know: High number (small aperture) = little hole/lets in less light. Low number (large aperture) = big hole/lets in more light. There’s a little more to know: Little hole = large depth of field. Big hole = small depth of field. We’re talking mainly about exposure in this post, though. Learn more about aperture and depth of field in What is Aperture in Photography? and Intro to Depth of Field in Filmmaking.
However, depth of field (DoF) and exposure do relate to one another. Think of your lens as simply a tube with a wide opening that starts at a low number, like 1. A bunch of light can pass through that wide circle but it also isn’t very good at focusing on anything. So let’s make that circle inside the tube a little smaller and raise 1 to 2. It focuses a little better on more of your scene but you are letting in a little less light, too. Now just think of that concept all the way up the number chain. Numbers get bigger and bigger and that hole gets smaller and smaller, letting in less and less light but focusing on more and more of your overall scene.
The quick takeaway for this one: your shutter is like a little door that sits in front of your sensor. Open the door and light hits the sensor. The longer you leave that door open, the more light that soaks into the sensor. The shutter speed is how fast that door can swing! More time open = more light. Less time open = less light.
On your camera, you will often see on your shutter dial numbers ranging sometimes from 1 or lower all the way to 1000 or more. This is a measurement of the amount of time the shutter stays open and lets light onto the sensor, 1 being 1 second and 1000 being 1/1000th of a second. Shutter speed affects more than just how long light is hitting the sensor. It also affects how much motion is being registered on the sensor. When you leave your shutter open and let in all that light, you are also “letting in” movement. It can be difficult to hand-hold your camera and keep your image sharp while leaving your shutter open for longer than 1/125th of a second. There is actually a whole little trick in photography to help you with this – read Understanding the Reciprocal Rule in Photography to Take Better Photos for extra credit.
While your shutter is open, think of it as “burning” the scene onto your sensor. If you are moving around while that scene is trying to burn, the lines are going to get blurred. Likewise, if your subject is moving around while that light is trying to burn in, you are going to end up with unsharp results with a lot of motion blur. Fast shutter speeds, where light is hitting the sensor for a shorter period of time, are good for reducing brightness and for freezing action. Yes, freezing action is as simple as increasing your shutter speed. However, it can come at the cost of exposure if one doesn’t compensate by either lowering the aperture or raising the ISO.
What Is a Stop of Light?
In order to adjust your exposure settings, you need to know how they are measured and displayed on your camera. But before we look at that, it is helpful to understand a common term in photography, which is a stop of light (commonly just called a stop – not to be confused with f-stop which is what we say for aperture settings).
Light is measured in lumens (total amount of light emitted) or lux (measure of illuminance). But that isn’t entirely useful when walking around trying to learn your camera and get exposed shots. Once you start looking at the exposure triangle and changing multiple variables that influence how bright an image will be, it’s far more helpful to have a way of comparing – proportionally – how bright the resulting image will be. That’s where stops come in.
One stop is equal to double (or half) the amount of light (depending on which direction you’re making the change). This is most easily illustrated with shutter speed. If all other variables remain the same, keeping the shutter speed open for twice as long will result in twice as much light hitting the sensor, making the image twice as bright. Thus, the image will be one stop brighter. If you cut the shutter speed in half, only half as much light will hit the sensor, and the image will be half as bright (or twice as dark). In this case the image is one stop darker.
Stops are exponential, multiplying onto each other. If you go one stop brighter you will double the amount of light. If you go another stop brighter you will double it again, so two stops brighter will be four times brighter than the original. Likewise, three stops brighter will give eight times the amount of light, four stops sixteen times, and so on.
Let’s look at how shutter speed, ISO, and aperture are measured to better understand stops of light.
How Shutter Speed Is Measured
Shutter speed is measured in time. As mentioned, shutter speeds will be in fractions of a second. For long exposure photography, you might measure it in full seconds, minutes, or even hours. For example, 1/100th of a second is a relatively common shutter speed because it is fast enough to freeze basic motion but slow enough to let in a fair amount of light, making it easier to get a proper exposure. If you increase the shutter speed to 1/200th of a second, you are cutting the amount of light in half. This is often referred to as one stop faster, but can be thought of as one stop darker. A shutter speed of 1/400th of a second would be one stop faster than 1/200th and two stops faster than 1/100th. So if you hear an instructor or workshop guide ask you increase or decrease your shutter by 1 stop, you now have a better understanding of what they’re talking about.
How ISO Is Measured
When the ISO standards for film were developed, they were set to make it easy to quickly identify how much brighter or darker one film is than another. Every time the ISO value doubles, so will the brightness of the image. Digital cameras followed this same convention. Most digital cameras have a base ISO of 100. Some might be lower or higher, but this is a good starting point. If you bump the ISO to 200, you will be shooting one stop brighter. An ISO of 400 is two stops brighter than ISO 100, ISO 800 is three stops brighter, and so on.
How Aperture Is Measured
Aperture values are the least intuitive. They are measured in f-stops, which are calculated based on the focal length divided by the diameter of the aperture itself. The result is a fractional number, with several specific fractions becoming standard f-stops for photographers.
While there is certainly a pattern to these f-stop values, it’s not easily identified and it’s best to just memorize the sequence of stops: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. The smaller/lower the f-stop value, the wider the aperture, the brighter the image. So if you start from f/5.6, f/8 would be one stop darker, f/11 two stops darker, and so on. Conversely, f/4 would be one stop brighter, f/2.8 two stops brighter, etc.
How to Apply the Exposure Triangle to Your Photography
Once you understand how the three exposure settings relate to one another, you can combine them into the exposure triangle and determine how to make adjustments to create the image you want. To start, you need to establish some baseline for a proper exposure. Do not change too many settings at once! It’s extremely helpful to “set it and forget it” with 1 or 2 out of the 3 settings so that you can really master one at a time. Know that you really want to take shots all day with nice, blurry backgrounds? Set your aperture to something very low, like f/1.2 or f/1.4, and forget it – obtain a good exposure using only ISO and shutter speed.
In fact, let’s use that as an example. You want bokeh for sure, so imagine that you are shooting a portrait and your baseline exposure settings came out to f/4, 1/100th of a second, at ISO 100. These are the settings you ended up with making adjustments enough so that the light meter in your camera is hovering around the middle. But you wanted to use an aperture of f/1.4 to completely blur the background! Okay, f/1.4 is 3 stops brighter than f/4 (remember f/4 > f/2.8 > f/2 > f/1.4). In order to not massively overexpose your image, you’re going to need to adjust the other settings to be 3 stops to be darker. Many cameras won’t go below ISO 100, so you need to adjust the shutter speed 3 stops faster to make the image darker. 3 stops faster would be 1/800th of a second (1/100 > 1/200 > 1/400 > 1/800). So your final exposure will be f/1.4, 1/800th of a second, ISO 100.
If thinking about stops of light intimidates you then don’t think about it so precisely. I certainly don’t walk around and change my settings by counting stops in my head. It’s just a tool. Usually it’s enough to just know that ISO has a range, with a quality cost to cranking it high. Slow shutter speeds let in a lot of light but they also make everything blurry. Low apertures also let in a lot of light but can make most of the scene fuzzy. This is usually enough to help you get your footing on weighing creative decisions with “correct” exposure out in the field.