Originally Posted by
Mike Buckley
It has to do with understanding the so-called exposure triangle: aperture setting, shutter speed and ISO value. When changing one or more of those three exposure settings, the others change in proportion when maintaining the same exposure.
You probably know that exposing the sensor to twice as much or half as much light is the difference of one more or one less stop of exposure, respectively. It's easy to remember that changing only the ISO to a value that is twice or half as high increases or reduces the exposure by one stop, respectively, by affecting the sensor's sensitivity to light. Similarly, changing only the shutter speed to a duration of time that is twice or half as fast reduces or increases the exposure by one stop, respectively, by affecting the amount of time the sensor is exposed to light. The mathematical relationship between aperture settings, which affect the amount of light exposed to the sensor, is far more complicated and I have never pretended to understand the underlying math behind the relationships. Even so, it's easy to memorize that the following settings are sequentially 1 stop apart: f/1.4 -- f/2 -- f/2.8 -- f/4 -- f/5.6 -- f/8 -- f/11 -- f/16 -- f/22. (Those of us who grew up on film cameras constantly saw those settings that we made directly on the lens. That made it easy to learn those relationships, unlike the experience of digital photographers who see every imaginable aperture setting displayed in the LCD.)
If you leave one setting unchanged and change one setting, you must change the third setting by the same degree in the opposite direction to maintain the same exposure. As an example, if you don't change the shutter speed and if you change the aperture by one full stop, the ISO value must also change by one full stop in the opposite direction to maintain the same exposure.
Most important: study up on the relationships in the triangle of exposure, which I'm sure is explained very well in one of the CiC tutorials. Understanding and using those relationships is fundamental to everything about controlling exposure beyond the realm of automated point-and-shoot photography.