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The Exposure Triangle comprises aperture, shutter speed, and ISO.
Aperture: controls the area over which light can enter your camera
Shutter speed: controls the duration of the exposure
ISO speed: controls the sensitivity of your camera's sensor to a given amount of light
A photographer can use many combinations of the above three settings to achieve the same exposure. The key, however, is knowing which trade-offs to make, since each setting also influences other image properties. For example, aperture affects depth of field, shutter speed affects motion blur and ISO speed affects image noise.
A camera's aperture setting controls the area over which light can pass through your camera lens. It is specified in terms of an f-stop value, which can at times be counterintuitive, because the area of the opening increases as the f-stop decreases. In photographer slang, when someone says they are "stopping down" or "opening up" their lens, they are referring to increasing and decreasing the f-stop value, respectively.
Every time the f-stop value halves, the light-collecting area quadruples. There's a formula for this, but most photographers just memorize the f-stop numbers that correspond to each doubling/halving of light:
Aperture Relative Light Example Shutter Speed
f/22 1X 16 seconds
f/16 2X 8 seconds
f/11 4X 4 seconds
f/8.0 8X 2 seconds
f/5.6 16X 1 second
f/4.0 32X 1/2 second
f/2.8 64X 1/4 second
f/2.0 128X 1/8 second
f/1.4 256X 1/15 second
A camera's shutter determines when the camera sensor will be open or closed to incoming light from the camera lens. The shutter speed specifically refers to how long this light is permitted to enter the camera. "Shutter speed" and "exposure time" refer to the same concept, where a faster shutter speed means a shorter exposure time.
Shutter speed's influence on exposure is perhaps the simplest of the three camera settings: it correlates exactly 1:1 with the amount of light entering the camera. For example, when the exposure time doubles the amount of light entering the camera doubles.
With waterfalls and other creative shots, motion blur is sometimes desirable, but for most other shots this is avoided. Therefore all one usually cares about with shutter speed is whether it results in a sharp photo — either by freezing movement or because the shot can be taken hand-held without camera shake.
Shutter Speed Typical Examples
1 - 30+ seconds Specialty night and low-light photos on a tripod
2 - 1/2 second To add a silky look to flowing water
Landscape photos on a tripod for enhanced depth of field
1/2 - 1/30 second To add motion blur to the background of a moving subject
Carefully taken hand-held photos with stabilization
1/50 - 1/100 second Typical hand-held photos without substantial zoom
1/250 - 1/500 second To freeze everyday sports/action subject movement
Hand-held photos with substantial zoom (telephoto lens)
1/1000 - 1/4000 second To freeze extremely fast, up-close subject motion
The ISO speed determines how sensitive the camera is to incoming light. Similar to shutter speed, it also correlates 1:1 with how much the exposure increases or decreases. However, unlike aperture and shutter speed, a lower ISO speed is almost always desirable, since higher ISO speeds dramatically increase image noise. As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren't otherwise obtainable. Common ISO speeds include 100, 200, 400 and 800, although many cameras also permit lower or higher values.
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