Photography Basics: Understanding Exposure
December 19, 2013
•IADT General, Photography
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When you're ready to move beyond automatic modes and point and shoot photography, it's time to look at your camera's settings and understand how they control the exposure of your photographs. Being able to manipulate every facet of the light coming into your camera is one of the key photography basics you need to know when taking your pictures to the next level.
The Exposure Triangle
Controlling your exposure requires you to juggle three different, yet interconnected settings — aperture, shutter speed and ISO — and each adjustment has its own effects on the quality of the image. Aperture determines how much light enters the camera, shutter speed determines how long that light reaches the sensor and ISO determines how sensitive the sensor is to that light. A change in one necessitates a change in one of the other two to maintain correct exposure. If you open up the aperture to let in more light, you must either shorten the time the shutter is open or decrease the ISO to maintain balance.
Aperture refers to the size of the opening of the lens. Larger apertures let in more light, smaller apertures let in less light. It's important to remember the relationship between the f-numbers and the aperture size are inverse — f/2.8 is a bigger opening than f/4. In addition to controlling light, the size of the aperture will determine the depth of field of the photograph, which refers to how much of the image in front and behind the focal point is in focus. A larger aperture has a shallow depth of field, blurring foreground and background. A smaller aperture has a deep depth of field with more of the image in focus. While not a direct effect of exposure, it's important to consider your depth of field when setting your aperture.
Shutter speed is the next leg of the exposure triangle, and also directly affects how much light enters the camera. It refers to the amount of time the shutter covering the image sensor or film remains open. The longer the shutter is open, the more light reaches the sensor. Shutter speeds are typically expressed in fractions of a second, with 1/250 letting in less light than 1/60. Shutter speed also impacts the quality of the image, as the lower the speed, the more likely camera shake or motion blur in moving subjects can become apparent. Slower shutter speeds—1/30 and under — are useful for conveying motion, like a flowing waterfall. Faster shutter speeds—1/60 and above — are excellent for freezing a fast moving subject without blur, like a speeding car.
The third leg of the triangle is ISO, or film speed if you're still an analog shooter. The ISO refers to the level of sensitivity to light of the image sensor, and the higher the number the more sensitive the sensor becomes and therefore the less light is needed to expose the image. An ISO of 400 is more sensitive than an ISO of 100. Just like the other legs of the triangle, ISO also has trade-offs in image quality. Turning the ISO up in dark situations can allow you to get a workable shutter speed and aperture combination, but higher ISO settings introduce noise to the image, giving it a "grainy" appearance.
Bringing it All Together
The key to using the exposure triangle is to match your settings to the subject. For example, when shooting sports, keep your shutter speed as high as possible to freeze action; use a larger aperture and possibly a higher ISO to achieve that, taking the trade-offs of the shallow depth of field and higher noise. On the other hand, when shooting landscapes you will want to maximize depth of field, closing your aperture down. This will mean longer shutter speeds and avoiding high ISO noise for image quality, requiring the use of a tripod or other support. It may seem like a lot to juggle at first, and it can be intimidating, but once you begin to master the photography basics of aperture, shutter speed and ISO, you'll see the improvements in your images in no time.
Photo credit: MorgueFile