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What is the Exposure Triangle – A Beginner’s Guide

Whether you are beginning your photography journey or have been at it a while, you’ve probably heard the term exposure triangle. But, what is the exposure triangle? The exposure triangle is made of three components; aperture, ISO, and shutter speed. These three components work together to create a well exposed image.

In this article:

Along the way I’ll provide images that, I hope, you will find helpful.

Without further ado…

BEHOLD! THE BALANCED LIGHT METER!

EXPOSURE TRIANGLE | CAMERA LIGHT METER SCALE

You are at a beautiful place and put the camera up to your eye. You likely see some information overlaid on the scene. The scale at the bottom of the display that looks similar to the image above is called the light meter. It displays how close to the “correct” exposure your image would be if you clicked the shutter at that moment.

You might be asking yourself, why did I start talking about the light meter? That is a good question. The answer is

If we are going to learn about the factors that control the exposure of a photograph it is a good idea to first understand the tool the camera gives us to evaluate the exposure.

There are some more advanced topics related to changing how the camera measures the light coming into the sensor, but that is outside the scope of this article. For the purposes of this article we’ll assume the light meter reading will give you a good exposure. The purpose of the following discussion is to investigate how the shutter speed, aperture, and ISO control the position of the blue marker on the scale. But first, let’s talk about the scale and what it tells you.

The numbers

0, +/-1, +/-2, +/-3 refer to stops of light. What is a stop of light?

A stop of light is either double or half the amount of light being allowed to reach the camera sensor.

Thinking about this another way, consider you have a five gallon bucket of water. This represents your current level of light reaching the sensor. If you increase the light by one stop, you would need a ten gallon bucket to hold the additional water. Likewise, lowering the amount of light one stop would mean you would pour out 2.5 gallons of water from your original five gallon bucket.

The subdivisions

The tic marks between the stops can be either 1/3 or 1/2 stops. Usually, camera manufacturers ship cameras with 1/3 increments selected. In my opinion, using 1/3 stop increments gives you a bit more refinement when adjusting settings. For this discussion we’ll keep things in terms of whole stops.

Exposure triangle

EXPOSURE TRIANGLE | KEY IMAGES

After reading about the light meter and stops, hopefully, you have a better idea of those concepts. They are important for our continued discussion of the exposure triangle.

As I mentioned above, the exposure triangle consists of three components; aperture, shutter speed and ISO. These three components work in concert to create a balanced exposure for a given scene. If you have a correct exposure and change one of the settings you need to change one or both of the remaining settings to once again balance the exposure.

Think of the light meter like a seesaw. If you have a balanced exposure the aperture, shutter speed, and ISO are in balance to let the right amount of light hit the sensor to capture the scene. In the following image I’ve shown ISO on one side with shutter speed and aperture on the other. This is not the only combination that works. It is just the one I chose to include.

EXPOSURE TRIANGLE | BALANCING THE SEESAW OF LIGHT

Shutter speed

Shutter speed is defined as the amount of time the shutter is open while exposing a given scene. Measured in seconds or fractions of a second, the shutter speed is probably the easiest component of the exposure triangle to understand. Put simply, the longer the shutter is open, the more light is gathered by the sensor and vice versa.

When you push the shutter button you hear a CLICK CLICK, and an image is displayed on the back of your camera. The first CLICK is the shutter opening to start letting light hit the sensor. The second CLICK is the shutter closing of the sensor from the scene. The duration of this process is the shutter speed.

Creative effects

Stopping action – Fast shutter speeds can stop the motion of a moving object. I captured the following image of a waterfall using a shutter speed of 1/250 seconds. This shutter speed was fast enough to freeze the water falling and flowing through the scene.

EXPOSURE TRIANGLE | WATERFALL FAST EXPOSURE

Slowing action – Slow shutter speeds can create blur for a moving object. In contrast with the image above, the following image blurs the motion of the water moving through the scene. This image was created by using a 15 second shutter speed. The resulting image from the same exact location looks significantly different don’t you think?

EXPOSURE TRIANGLE | WATERFALL LONG EXPOSURE

Aperture

Aperture, like shutter speed makes a physical change to a part of the camera, namely, the lens. Within each lens are a series of blades that control the diameter of the hole through which the light of the scene can travel to reach the sensor. If that sounds confusing, think about looking at yourself in a mirror. Start by turning off the light for a few seconds, then turn it on and watch what happens to your pupils. They should start large, then get smaller. If you do the process in reverse your pupils should get larger. This happens because your pupils are what control the amount of light reaching the parts of your eye that your brain turns into what you see. Pretty cool, right?

So, in each of our lenses we have the ability to control the diameter of the opening. This opening is referred to as the “aperture” and measured in “f stops”. (see the wiki description here)

Creative effects – depth of field

Changing the aperture not only controls the amount of light reaching the sensor, it also controls the depth of field. The depth of field means the amount in front of and behind the subject that is in focus. The easiest way to understand this concept is visually.

In the two following images you’ll notice the background detail change from a blurry blob to a more textured appearance. The first series compares one stop increments from f4.0 to f22.0. The second is a side-by-side comparison of f4.0 and f22.0.

***Notes: The positions of the camera, subject, and background remained constant. Changes made to the aperture were balanced by changing the shutter speed to maintain a consistent exposure for the scene.

EXPOSURE TRIANGLE | DEPTH OF FIELD COMPARISONS
EXPOSURE TRIANGLE | SHALLOW VS. DEEP DEPTH OF FIELD

ISO

ISO is a measure of the light sensitivity of a digital sensor or film. In the days of film you would buy different ISO rated films for different scenarios. If you shot roll film, you were stuck with that ISO until you changed rolls. Today’s digital cameras have the advantage of being able to change the ISO value from one shot to the next.

ISO is measured by a series of numbers, ranging from 50 to 32,000+ (these depend on camera model). The lower the value, the less sensitive to light. Conversely, the higher the number the higher the sensor or film’s sensitivity to light.

Why is this important? Light sensitivity determines how well the camera can see a scene. Think about your camera sensor or roll of film like your eyes.

In the middle of the day you can see everything around you. If it is too bright, what can you do? Put on a pair of sunglasses, right? In this situation your eyes are too sensitive. Putting on the sunglasses is akin to lowering the ISO.

Now, what about the middle of the night? Your eyes are not sensitive enough to light to see well in the dark. If you don’t have a light with you there are few options. One is to stay out long enough that your eyes become somewhat adjusted. Another would be to put on a pair of night vision goggles. The goggles amplify the ambient light to give you the ability to see in the dark. This is like raising the ISO value on the camera. You are increasing the sensor’s sensitivity to the ambient light. Unfortunately, all sensors have a limit to their sensitivity. Once crossed, the images begin to suffer from a phenomenon known as noise.

Noise

Noise is a term used to describe the graininess of an image. Because lower ISO values are less sensitive to light, the images produced will show little-to-no noise. However, as ISO increases, the greater chance your images become grainy, or noisy. This becomes a concern for printing and displaying photos because they will lose sharpness and clarity.

Let’s take a look at some examples of noise. The first image below shows a series of exposures at different ISO values. The lowest, ISO 100, progressing up to ISO 25600. The second image highlights the difference between the noise levels of ISO 100 and 25600.

***Note: All images were processed exactly the same.

EXPOSURE TRIANGLE | IMAGE NOISE COMPARISON FOR VARIOUS ISO SETTINGS
EXPOSURE TRIANGLE | IMAGE NOISE COMPARISON FOR ISO 100 VS IS0 25600

WOOF – THAT’S SOME NOISE, EH?

The cool part about ISO

Today’s digital cameras are pushing the boundaries of light sensitivity. This allows photographers to capture scenes that wouldn’t have been possible a decade ago. The new sensor technologies make it possible to photograph the night sky with sharpness and clarity. Here are a couple examples from my travels.

EXPOSURE TRIANGLE | LAKE TAHOE – WARMING UNDER THE MILKY WAY
EXPOSURE TRIANGLE | BELIZE STAR TRAILS
EXPOSURE TRIANGLE | MOBIUS ARCH, ALABAMA HILLS

Component relationships

To this point we have talked about the components of the exposure triangle separately. In order to finish this discussion it is important to examine how they interact with one another to maintain a balanced exposure.

EXPOSURE TRIANGLE | BALANCING THE SEESAW OF LIGHT

Returning to the seesaw analogy from above I’ve chosen to place ISO on the left with shutter speed/aperture on the right. Let’s take a look at two examples of how changing ISO is balanced by shutter speed and aperture, respectively.

EXPOSURE TRIANGLE | BALANCING INCREASED ISO WITH SHUTTER SPEED

In this example we keep the aperture constant. As you can see, increasing the ISO requires an increase in the shutter speed to maintain balance. Why is this true? As ISO increases the sensor becomes more sensitive to light. If the sensor is more sensitive to light, then less light is required to capture the image at the same exposure as the original settings. Therefore, to counteract the higher sensitivity, the shutter must be open for less time. This means the shutter speed increases (the time from open to close is less).

EXPOSURE TRIANGLE | BALANCING INCREASED ISO WITH APERTURE

For this example we hold shutter speed constant. Again, we increase the ISO, but the aperture required decreases, not increases. Why is this true? As we said above, the increased ISO means the sensor is more sensitive to light. Therefore, to maintain the original exposure we must decrease the aperture of the lens to let less light get to the sensor.

Remember, decreasing the aperture means reducing the pupil diameter of the lens (using human eye analogy). This is known as “stopping down”. F4.0 to f5.6 is an example of stopping down. “Stopping up” refers to enlarging the diameter, thus allowing more light to reach the sensor. F11.0 to f8.0 is an example of stopping up.

Aperture and Shutter Speed Comparison

Finally, let’s take a look at the relationship between aperture and shutter speed using the depth of field experiment from earlier. The aperture decreased from f4.0 to f22.0, with a constant ISO. As you’ll see in the chart below, the shutter speed needed to increase to maintain the same exposure.

EXPOSURE TRIANGLE | DEPTH OF FIELD COMPARISONS
EXPOSURE TRIANGLE | BALANCING EXPOSURE FOR DEPTH OF FIELD EXAMPLE

Thank you

Thank you for reading this article. I hope you found the information useful. If you did, please check out my previous articles on choosing your subject and ways to compose your scene. As always, if you have any questions, please drop a question in the comments below. See you on the trail.

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