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Exposure Triangle

Jun 21, 2012
Maureen Spagnolo

Photography is about capturing light. Without light, there is no image. Since the invention of photography, the image captured on light-sensitive material has been called an exposure. Correct exposure is one of the most important factors of an image.

When we set our digital cameras on automatic mode, they choose three things for us: the aperture size, which lets in the light; the length of the exposure, which is how long the shutter stays open; and the ISO setting, which is the speed, or sensitivity to light.

Deep depth of field – to have the whole scene in focus needs a small aperture: f/22, ISO 200, 1/100. A tripod is recommended for scenic shots.
Deep depth of field – to have the whole scene in focus needs a small aperture: f/22, ISO 200, 1/100. A tripod is recommended for scenic shots.

Sometimes, the automatic setting is fine, and other times it is not. [Editor's Note: ISO literally stands for "International Standards Organisation". It refers to the industry norm for the sensitivity of emulsion based film.] Learning to control our settings enables us to capture the images we see, as we see them. A correct exposure has a perfect balance between aperture, shutter speed, and ISO.  But, there are many variations possible.

A shallow depth of field requires a wide aperture: f/2.8, ISO 100, 1/125. (background is blurred)
A shallow depth of field requires a wide aperture: f/2.8, ISO 100, 1/125. (background is blurred)

The viewfinder has a series of numbers on it – or they’re on the lens. We might see 4, 5.6, 8, 11, 22, and 32. On a point-and-shoot style camera, these settings may not go past 8. Each number corresponds to an opening of the lens, called an f-stop. These lens openings control the volume of light reaching the sensor during exposure. The smaller the number, the smaller the opening – allowing in less light.

The shutter speed determines how long the lens is left open for each exposure. The numbers represent fractions of a second. Thus, the shutter is open for longer periods of time with smaller numbers such as 1/500, 1/1000. As we decrease the numbers, such as from 1/500 to 1/250 – we increase the amount of light entering the camera.

The final exposure leg, called the ISO, can be likened to ants on a donut. 200 ants/ISO will get the job done quicker than 100. If we change our ISO setting from 100, to 200, then double the amount of light is let into the sensor – which allows us to decrease our aperture size, or increase the shutter speed, to achieve a correct exposure.

Low light shot. ISO 6400, f/2.8, 1/15 sec. Slow shutter causes moving subjects to blur.
Low light shot. ISO 6400, f/2.8, 1/15 sec. Slow shutter causes moving subjects to blur.

If we need all of the scene to be in focus we need a deep depth-of-field. This translates to using a smaller aperture - as small as we can get it (which means a larger number) and still achieve correct exposure. Yet, if we want a close-up of an ant, and want the rest of the ants to fade out, we'll use a wide aperture of f/4, or wider, (which means a smaller number), to get a shallow depth-of-field. This also gives us more light on our image, and requires an adjustment of shutter speed, or ISO.

Stop action: fast shutter speed. 1/1000 sec. Other settings adjusted accordingly (ISO 100, f/8)
Stop action: fast shutter speed. 1/1000 sec. Other settings adjusted accordingly (ISO 100, f/8)

Sports and moving subjects mandate a fast shutter speed to stop action – for example, 1/500 and faster. If we're after a blurred action effect, we need a slow shutter speed, such as ¼ of a second, or slower. This makes a great effect when photographing flowing water, though a tripod is a must for such shots.

Moving our ISO up to higher numbers allows us to capture faster moving subjects, or, subjects in low light. But there's always a trade-off – which, in such cases often results in more “noise” in the image – small artifacts that often create an undesired effect.

Each of the settings affects the others, and the more we understand, the more confident and in control of our photography we become.

We learn by doing – thus, it's a good idea to turn off the automatic setting on our camera, and practice shooting with the exposure triangle in mind to get the effects we want.

Maureen Spagnolo is a photographer, living in Washington, DC. She writes on a variety of social issues in addition to her photography articles.

Her passion for photography began in her twenties when she fell in love with a photographer, and then took her first photography class. She used any camera she could get her hands on - until she got her first DSLR camera several years ago. Since then, she has gone gangbusters - taking photos like a junkie (!) and reading everything about photography she could find. She now owns enough (never enough!) cameras, lenses, tripods, and camera paraphernalia to warrant an add-on to her home owner's insurance.

Yet, photography is not about equipment. It's about passion, and seeing - qualities that can turn a cell phone into a tool which can compete with the most expensive camera.

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