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Picture This: The Exposure Triangle

Lightness and darkness depend on the right balance of shutter spedd, aperture and ISO.

By Earl Nottingham

At some point, beginning photographers will inevitably venture from the safe haven of the camera’s automatic settings to the world of manual settings, where the three elements that control exposure must be individually considered and changed in order to obtain a proper exposure.

The “proper exposure” for any image can be very subjective. Darker-than-normal exposures often convey a moodier, more somber look to a scene, whereas a lighter exposure may give an ethereal feeling. In the days of slide film it was common to shoot several images of the same scene with different exposures (bracketing) in hopes that one would be more aesthetically pleasing.

A simple graphic known as the Exposure Triangle serves to illustrate not only how the three individual yet interrelated aspects of exposure (shutter speed, aperture and ISO sensitivity) affect the final exposure, but also why the photographer might want to prioritize one over the others. A change to any one of the three settings will make the image darker or lighter. The key is to determine which of the three is most important to your shooting situation, and then prioritize and apply the appropriate shutter speed, aperture or ISO to get a proper exposure. You can easily see how adjusting any setting affects exposure by looking at the exposure scale in the viewfinder or LCD screen and noticing the changes.

Let’s look at the three exposure controls depicted by the Exposure Triangle and see how each not only affects the darkness and lightness of a photo but also influences the overall look and feel.

ISO speed is a numerical reference to the sensitivity of the sensor — its ability to gather light. Traditional films had fixed sensitivities such as ISO 100 or ISO 400 — unlike a digital sensor that can have variable sensitivities from very slow to very fast. A faster ISO film or sensor needs less light to register the same exposure than a slower ISO film/sensor. Traditionally, the price you paid for a speed gain in the higher ISOs was an increase in digital noise — the equivalent of film grain. These days, however, this is less of an issue because of the quality sensors being produced. It’s not a stretch to now shoot in very low light at ISOs of 3,200 through 12,800 — or faster.

In practice, you will generally set an ISO and leave it there for the given lighting situation and then use your shutter speed and/or aperture as your primary exposure controls. Probably 80 percent of my photography is done with the ISO locked on 160, a throwback to the days when I only shot ISO 100 transparency film.

The aperture, denoted by a numerical f-stop, refers to the opening of the lens, and, like the iris of the human eye, it can open and close in response to the intensity of light. The aperture becomes the primary exposure control when your image priority is for either a very shallow depth of field (wide aperture) for situations such as portrait photography where a soft background is preferred, or for a wide depth of field (small aperture) for landscapes where the foreground and background are sharply in focus. For portraits I generally shoot around f/2.8, whereas most landscapes will be at around f/16.

Remember, any change in the aperture will require a corresponding change in the shutter speed to maintain the same overall exposure. Just recenter your meter by changing the shutter speed.

If stopping the motion of your subject is important, then shutter speed becomes your priority. Fast-action subjects like sports require fast shutter speeds at 1/500th of a second or faster — especially if you aren’t using a tripod. Since the aperture setting (f-stop) is of secondary concern in these situations, you want to manually set the fast shutter speed and just choose whatever f-stop will center the meter. Conversely, if you desire a slow shutter speed for situations such as the blurring of waterfalls for that silky, flowing look, then you would want a shutter speed of around 1/15th of a second or slower. At these slow shutter speeds, a tripod is a must unless you are trying to obtain an artistic camera motion effect.

Without a doubt, shooting with manual settings does require more time and conscious thought. With practice, however, it becomes second nature and ultimately leads to greater creative control.

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For more on TP&W magazine photography, go to our Photography page


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