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.”

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.

Light

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:

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.

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.

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).

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.