What is an Exposure? :: Digital Photo Secrets

What is an Exposure?

by David Peterson 5 comments

You’ve almost certainly heard someone refer to the act of taking a photograph as “making an exposure.” But what does the word “exposure” actually mean? Read on to find out.

Originally, “exposure” had a pretty literal meaning—photographic film is light sensitive, so to take a photograph you simply “expose” the film to light for a brief period of time, a process which results in a photograph. So in simple terms, the exposure is the image that is recorded when your camera’s shutter opens, thus exposing the film (or today, your camera’s image sensor) to light.

The word has evolved since then, and you may hear it used to refer to any number of photography-related subjects. For example, you’ll hear people talk about exposure in terms of the quantity of light that was captured—“underexposure” means that there wasn’t enough light, and “overexposure” means that there was too much. You may also hear people refer to a printed photograph as an “exposure.” And if you still shoot film, you’ll also hear people talk about how many exposures are on a roll.

The exposure triangle

There are three primary variables that go into creating an exposure. You’ll hear these three variables referred to collectively as “the exposure triangle,” because each one is just as important as the other, and each one impacts the exposure in a different way.

Shutter speed

The first of the three variables is shutter speed, which is used to adjust the amount of time that your camera’s image sensor is exposed to light. For example, if you take a picture using a shutter speed of 1/60th of a second, that means that your camera’s shutter is open for exactly 1/60th of a second. That very brief period of time is enough for your image sensor to capture and record the photograph, provided that the other two variables are correctly adjusted.

Shutter speed does more than just let light into your camera—the length of time that the shutter remains open can also affect what your image looks like. For example, very fast subjects shot using very fast shutter speeds will appear frozen in time. Even water droplets can appear tack-sharp in mid-air, provided you choose a shutter speed fast enough to capture them that way. On the flip side, if you use a slow shutter speed you will capture motion blur. Sometimes this is undesirable, but motion blur can be used creatively, too. For example, very long shutter speeds can capture star trails or car light trails, which will appear as long streaks in the final image. This can add a lot of creative interest to a photograph.

Slow shutter speeds are only a problem if there is movement in the scene and/or you haven’t mounted your camera on a tripod. Trying to hand-hold your camera during a long exposure may cause “camera shake,” or visible blur caused not by the movement of your subjects, but by the movement of your hands. Likewise, if your camera is mounted on a tripod you can take long exposures of just about any stationary object—but anything that moves will still show up as blur even if your camera is stabilized.

Aperture

Your aperture serves the same basic purpose as your shutter speed does, in that it regulates the amount of light that reaches your camera’s sensor. Aperture basically just refers to the width of the diaphragm between your lens and the image sensor—a small aperture can be a pinhole size while a larger aperture can be the size of a walnut. The larger the aperture, the more light reaches your camera’s sensor.

Aperture differs from shutter speed in that it is related to the size of the opening that lets light reach the sensor rather than the amount of time that the sensor is exposed to that light. So a larger aperture will let you take photos in lower light, and a small aperture will let you take photos in bright light—but there’s a lot more to it than that. Large apertures let you use fast shutter speeds because when the opening is large you need less time to get the required light to the image sensor. Small apertures, by contrast, require long shutter speeds for the opposite reason—there’s less light overall going through that opening, so the shutter needs to be open for more time.

Aperture also has one additional, but critically important function—it controls the depth of field in your photograph. Depth of field is a term you’ve almost certainly heard, but if you’re new to photography you may have only a fuzzy idea about what it means. Simply put, depth of field is the amount of your photo that remains in focus. A photo with shallow depth of field may have an area of only a few inches where there is tack sharp focus, while a photo with broad depth of field may have crisp focus from foreground to background.

Now at first it may seem a little hard to understand why this happens, and it is actually a bit technical. I’m oversimplifying it here, but the depth of field in an image has to do with the many directions of the light. The light in any given scene doesn’t come from a single direction, but from all over the place—from multiple directions and angles. Your lens focuses those rays on your camera’s image sensor, but it doesn’t do a perfect job with all of those different light rays. When you make the aperture smaller, you’re limiting the rays that actually reach the sensor to only those that converge sharply. That’s why you get an image that is sharp all the way from foreground to background. When your aperture is large, on the other hand, all those rays are reaching your sensor—those that are sharply focused as well as those that are not, so you only get sharp focus in one small part of the scene.

ISO

The third element in the exposure triangle is ISO. ISO is actually an old term, and it stands for “International Standards Organization,” which probably makes no sense to you because it’s really not meant to. The term comes from the early days of film, when there was no standard for how sensitive to light film manufacturers would make their product, so photographers really had no easy way of knowing which films were suitable for low light and which films would be better for small aperture work such as landscapes. Thankfully, in 1974 light sensitivity was standardized for all film makes, which meant that a roll of Kodak 100 was roughly equal to a roll of Fujifilm 100, at least as far as light sensitivity was concerned.

Obviously most of us don’t shoot film anymore, but the term is still used to describe light sensitivity. Today you don’t need to switch out a roll of film in order to change the light sensitivity—you just change the setting in your camera’s menu system. That means you can shoot one frame in very low light and then step outside and use the next frame to shoot a sunny day landscape.

What’s important to you is how ISO affects your image. You already know that it makes your camera more sensitive to light, so one of the most valuable things it allows you to do is take photos in low light without the use of a tripod. High ISOs can also help you achieve a fast shutter speed in semi-bright conditions, which is important if you’re trying to stop very fast action in less-than-optimal shooting conditions.

But high ISOs have drawbacks, too, especially in older model cameras. When you turn up your ISO too high, you can get digital noise in your photos, which shows up as a sort of sandy or grainy texture. Very high ISOs may produce images with “banding” or long streaks of discoloration, and noise that actually looks pixelated. Less obvious are the problems high ISOs may cause with color and tone, which can appear progressively muddy or poorly defined as ISO increases. Now, as I said this is really more of a problem for older model cameras—new models handle high ISOs very well and you may find that you can’t easily tell the difference between a photo you shot at ISO 400 and one you shot at ISO 3200.

  • Nikon D90
  • 3200
  • f/1.8
  • 0.02 sec (1/50)
  • 50 mm

Noisy Gordon by Flickr user sneeu

Conclusion

Each of those three factors combine to create an “exposure,” which is just photographer speak for a photograph. So you can think of an exposure as the end result of the different ways that your camera presents light to the image sensor. First, there’s the size of the diaphragm, or the opening between your lens and the image sensor. Then, there’s the length of time that light is allowed to pass through the diaphragm, and finally, there’s the overall sensitivity of the image sensor to the light that arrives. Those three factors combine to create the exposure, or the image that you capture in pixels.

Summary:

  1. Definitions of “exposure”
  2. The exposure triangle
    • Shutter speed
    • Aperture
    • ISO

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Comments

  1. Winston says:

    Another article well explain, it is said simplicity is the best way to explain a subject. I was once a point and shoot person as my job called for ti. Sitting in a car for 10 to 15 hours to get a picture of a subject, point and shoot was the best way.Then the digital age came in and we had to learn all over again. My pictures are of a better quality now, but I am sure I can make them better with more practice. Thanks David for the help..

  2. Charles Williams says:

    Well done,Keep up the good work for us.Thanks!!!

  3. Nicolas Ramos says:

    This well described what it means exposure

  4. Mark Lee Clark says:

    I often get responses that my photos are under or over exposed when to me they look right. How can you tell what is the right exposure?

    • David Peterson says:

      Hi Mark,

      I think that's a subjective thing. If you're happy with the photos, then I wouldn't change them!

      If you want to be objective, you could look at the histogram of your images. If the graph is skewed to the left, it's likely to be underexposed. If it's skewed to the right, it could be overexposed. See more here: http://www.digital-photo-secrets.com/tip/785/histograms-deciphered/

      David.

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Difficulty:
Beginner
Length:
12 minutes
About David Peterson
David Peterson is the creator of Digital Photo Secrets, and the Photography Dash and loves teaching photography to fellow photographers all around the world. You can follow him on Twitter at @dphotosecrets or on Google+.