The three legs of the exposure stool are ISO, aperture, and shutter speed. These three variables work together to control the light that records an image. Shutter speed and exposure work in a reciprocal way–as one goes up, the other goes down. For example, when your exposure calls for a shutter speed of 1/125 and an aperture of 5.6, the same exposure can be achieved with a shutter speed of 1/500 and an aperture of 8.
This reciprocal relationship is helpful when you want to blur out the background of a photograph to emphasize the subject. By shifting to a larger aperture like 2.8 to decrease the depth of field, you then decrease the shutter speed to keep the proper exposure. The reverse is also true: if you need a faster shutter speed for fast-moving action, you can decrease the aperture opening and thereby use a faster shutter speed. Pretty neat, huh?
Higher ISO settings are needed when you reach the limit of aperture or shutter speed. When there isn’t enough light, there is a point when you cannot open up the aperture or decrease the shutter speed any more. The solution is to increase the ISO.
The three legs of the exposure stool work together to allow a variety of creative ways to get proper exposure.