Understanding Exposure, Part 1: The Exposure Triangle


Photography is all about capturing light. In fact, the etymology of the word “photograph” is basically “light drawing.” To make a photograph that we can see, we have to control both the amount of light that is exposed to a photosensitive surface, be it film or a digital sensor, and also control the sensitivity of that surface to the light. In this series, we will discuss a bit of the physics and characteristics of light and then how a camera and lens combine to control exposure by using what is commonly known as the “Exposure Triangle.”

Exposure Triangle

The Exposure Triangle comprises aperture, shutter speed, and ISO. These three camera and lens controls work together to regulate the amount of light that makes it to the light-sensitive surface (aperture and shutter speed) and the sensitivity of that surface (film or digital ISO). Not only do those three controls affect the light of a photograph, but they also have unique “side effects.” Aperture controls depth of field, shutter speed can blur or freeze action, and ISO can add or subtract film grain or digital noise from an image.

Each leg of the exposure triangle is going to get a separate article but, before we analyze the three sides of this virtual polygon, we need to establish a photographic foundation with light and exposure.


Exposure can be defined as the amount of light that falls onto the camera's light-sensitive surface. In any given scene, regardless of whether there is natural or artificial light being emitted, there is a measurable amount of light that illuminates your subject.

This amount of light varies due to four basic factors: intensity, duration, distance between light source and subject, and modifications to the light. This is not going to be a dissertation on light, but let's touch on some basics and those four factors before talking about controlling exposure.

Light is fascinating in that it behaves with the properties of both waves of energy and particles. This wave-particle duality affects the way light behaves inside and outside of a camera and lens. Let’s look at intensity, duration, distance, and modification of the light:

The sun, photographed from Manhattan with the DayStar Camera Quark and post-processed in Lightroom
The sun, photographed from Manhattan with the DayStar Camera Quark and post-processed in Lightroom

Intensity, the brightness of the light: A light source emits photons and, the more photons that are emitted by a light source, or reflected by an object, the brighter it is. A brighter photograph is created from a sensor or piece of film that has been hit by more photons than a darker photo. A darker image was exposed to a lower number of photons than the brighter image.

Duration: The sun is a constant light source, but you can escape the light by riding the Earth as it rotates away from the sun—or simply by going inside! Artificial light can be turned on or off and some is emitted in a short-duration flash. If you increase the amount of time that a given light is emitted from a light source, you can increase the number of photons that are collected by the camera.

Distance: Photography, unfortunately for some of us, involves mathematics. This lesson on exposure cannot escape math’s pull. For those of you with arithmetic skills like mine, I apologize in advance. The closer to the light source, the more photons you can capture with a camera. The farther away you are, the fewer photons you can collect. Easy, right? Well, what if you double your distance from the light source? There should be half the photons and half the light, correct? Nope. Thanks to something called the Inverse Square Law, you get 1/4 of the light when you double the distance. Why? This is because we are talking about area, not just distance. As light is emitted from most sources, it spreads (lasers are an exception). So, a light bulb at 5 feet appears 4 times as bright as it was at 10 feet. Similarly, a fictional planet orbiting our sun at a mean distance of 186 million miles gets only 1/4 of the sunlight that we enjoy on Earth at 93 million miles.

The Inverse Square Law, applied to light

Modifications: There are innumerable numbers of light modifiers that help control and shape artificial and natural light. You cannot dim the sun, but the clouds certainly can. You can also have your subject move into the shade—or you can create shade. Reflectors, diffusers, and gels are just a sliver of the available tools you can use to modify light.

Measuring Light

OK, now that we know how the amount of light can be altered, we need to assign a quantitative value to light so that we can measure its intensity, adjust our camera settings accordingly, and then adjust them further to brighten or darken an image. It is this image adjustment that leads us to the mathematical concept of "exposure value" or EV; sometimes referred to as "stops."

The intensity of light is its luminance, but, even with a number assigned to luminance, we really aren't interested in quantifying that because cameras can capture images in all kinds of light, or even in darkness. What we do care about is setting a baseline so that when we change camera settings we are aware of how the changes will affect the exposure and how to compensate if compensation is desired.

Photos taken with the same shutter speed at different apertures (note differences in depth of field)

Simplified, a "properly exposed" image can be given the baseline of EV 0. If we change something on the camera to make the image darker, we venture into a minus EV. Brighter is a positive EV. This is where the previously mentioned quantitative value comes in. EVs are given numbers so that we can measure the change from the baseline EV.

Why do we care about EVs? Well, you'll find out by reading on...

The goal in creating an exposure is to allow a specific amount of light into your camera and lens to capture your subject in a way that matches your artistic vision. Note that I did not say that the goal is a "proper exposure" and I have twice now used quotation marks around the phrase. Photography is art, and if you want to alter the image to be brighter (overexposed) or darker (underexposed) to better express your artistic vision, then never think that every frame you shoot needs to meet the definition of "proper exposure." It does not. I will keep using the word "proper," but do not read deeply into the term and feel free to add your own "air quotes" when you read it!

So, what you want to do is set up your camera and lens to allow the correct (for you) amount of light into the system to create the image you want. To control this light, you have the ability to adjust three separate settings inside the camera—the Exposure Triangle. As we mentioned before, there are two ways to control the amount of light that enters the camera and exposes the photosensitive surface (aperture and shutter speed) and one way to control the sensitivity of that surface (ISO).

Light's journey through the camera lens to the sensor

One way to simplify these adjustments is to compare the camera to certain elements of the human eye. Aperture functions like the eye's iris that opens and constricts the diameter of its opening to limit the amount of light allowed into the eye. Shutter speed is similar to blinking, except the eyelid is usually open when we are awake. However, if you can imagine your eyelids opening momentarily to capture a single image before closing, that is like a camera's shutter. And, finally, ISO is similar to the sensitivity of the rods and cones at the back of the eye.

It is important to know that in almost every camera that has a variable aperture, shutter speed, and ISO, there is a way to control these settings manually. Of course, you can find adjustable aperture rings and shutter-speed dials on older cameras and lenses for SLRs, but you can also likely control aperture, shutter speed, and ISO on today's point-and-shoot cameras, as well. Learning how and when to adjust these settings can help improve your photography, as it will give you more control over your images.

In Part 2 of this series, we will start by talking about aperture.


If you already feel overwhelmed by the amount of information above and are not mentally prepared for three more articles, here are the quick-and-dirty basics:

Exposure is controlled in a photograph by the camera's aperture, shutter speed, and the ISO of the film or digital sensor—the Exposure Triangle.

Aperture is the size of the opening of the lens. The larger the opening, the more light gets through. The smaller the opening, the less light gets through.

Shutter speed is a measurement of how long the shutter is open, allowing light to get to the film or sensor. The longer it is open, the more light reaches the film or sensor. The shorter the duration of time it is open, the less light reaches the film or sensor.

ISO is a measure of the sensitivity of the film or digital sensor to light. The higher the ISO, the more sensitive the surface is to light. The lower the ISO, the less sensitive.

Aperture also controls depth of field and shutter speed can freeze or blur action. ISO creates film grain or digital noise when increased.

All three of these controls are adjusted individually or in unison, to manage the brightness and capture of an image.


Master class for the novice in one article.  Great work!

Thank you for this article Todd! It has really changed my view on exposure on cameras :)

Hi you,

Thank you for reading, you!

Please let me know if you have any questions as you explore photography!



Years ago, I found the formula for calculating exposure value in a library book; I think the title is "Photometry". I programmed my Hewlett-Packard HP-67 calculator to calculate EV given shutter speed, aperture, and ISO. From a reading of light reading from the camera's meter or a handheld light meter, it would do the calculations and give the equivalent exposure setting if shutter speed or aperture was changed; I also incorporated Ansel Adam's Zone System in it. I later migrated the application to the HP-41C.

Wikipedia has a good article on Exposure Value; search for that.


Exponentian (^) takes precedence over division. ISO is not part of the formula, but I had to incorporate it to make it useful for photography. Log2 is base 2 (log x/log 2).

PS: I have a BS degree in Computer Science and when I see a personal need for an application, I'll create the application for myself. For my film photography, I label each roll as YYYY-Roll#. YYYY is the year (actually when I started in 1980, I didn't think about Y2K), and Roll# is a sequential number starting with 1 for each new year). Each film roll gets its own directory and I number each file to the frame number. But I found with Lightroom, I'd end up with multiple 01's, etc, without any indication of what roll it belongs to. I wrote a Windows application to prepend the directory name to the file name so Lightroom could sort it by file name.

Hey Ralph,

Good stuff!...although you have probably read about my affinity for math in many B&H articles! :)

Thanks, as always, for reading!



Hey Todd, I’m just an amateur photographer and I have enjoyed reading many of your articles. Thanks for breaking it down for a simple minded guy like me. I think about the triangle like this. If you make an adjustment to one side of the triangle it will affect one or both of the other two sides. That’s how a simple minded guy like me uses the triangle. Keep up with the articles, I throughly enjoy them.

Hey Dan,

Thanks for the kind words! I am glad you are enjoying the articles!

The biggest reason I break these concepts down is because I am simple minded, too! :)

Thanks for reading and please let me know if you have any questions as you explore photography!



Love to see you guys drop the triangle concept. It does not hold up in practice. This guy explains it well: [Link removed. YouTube Video: 6 Reasons Why We Should Ditch the Exposure Triangle.]

Hi Matt,

Thanks for the comment.

I have a lot of respect for John Hess and I don't disagree with any of his obviously accurate knowledge in his video.

Having said that, I would counter that the Exposure Triangle is an accurately simplified way to explain the three factors that control your image exposure.

Yes, you can poke holes in it if you get wrapped around the idea of assigning values to the triangle (Should it be the 3 Pillars of Exposure instead?), but, for a beginner, this is the most simple way to teach exposure that I can think of.

And, as someone who teaches photography in a college, I can tell you that ditching the exposure triangle in order to get granular with different concepts while adding things like ND filters, speed boosters, signal-to-noise ratio, raw processing, ISO invariance, and noise reduction would make my student's heads explode in the Introduction to Photography class!

The core of exposure is aperture, shutter speed, and ISO—three adjustments/concepts that can certainly be drilled down on to the Nth degree, but more intimate technical knowledge of those concepts is not really needed to understand exposure and make great images.

Thanks for reading!




Hi Todd. You adressed the Light Modifiers in te "Light" part of your excelent explanation.

"This amount of light varies due to four basic factors: intensity, duration, distance between light source and subject, and modifications to the light".

The variations in the sensitivity to light by sensors or film ("reciprocity") and the uses of filter or other light modifiying procedures or factors was well described decades ago and since You wrote about de BASICS of exposure (less well known than one could guess) you don't need to even mention them. No place to show off. :-)

Hi Roberto,

Thank you for the kind words and the morning humor. :)

And, thank you for reading Explora!



there is no clear, and correct, answer for ISO, Which controls or impact "THE EXPOSURE" of one picture, which was exposed.

Hi Grmurugan,

Thanks for your thoughts and thanks for reading!



I have multiple concerns regarding the exposure Triangle.  It seems inconsistent and flawed. Based on the direction of the arrows, sharpness increases as the shutter speed is slowed. That isn't correct. It also shows less light at a lower f-stop and more light at f/32? I'm really confused to the point of frustration. Please help.

Hey Evan,

Sorry to frustrate you! I can see why you got thrown here.

"Sharpness" is probably not the best word for the vector under the Shutter Speed portion of the triangle. I am going to see if our graphics folks can change it to "Motion."

And, you are correct, the "More" and "Less" light icons are reversed on the Aperture side of the triangle. We will have that fixed as well.

I apologize for missing these items on the graphic. Please follow up with me if you have more questions and thanks for pointing out our errors here!

Im new to this videotaping, I'm 56 yrs old and use to picking up a camera not a video recorder.  How do I determine Exposure- if its sunny should it be a minus and cloudy use a plus. Totally confused

Hi Linda,

Thanks for your question. I am a bit confused by your question. Are you asking about using exposure compensation?