How the ISO, the aperture, and the light conditions are related? How do they affect the shutter speed?
It's a little bit complicated. So, we'll make use of metaphor.
Let's start with basic of image capturing process. The process starts when the camera opens the shutter for light to move inside through the aperture. When film or sensor of the camera get enough light, the shutter will be shut. It's not that complex, right? We hope you agree.
Now, we'll use the water as a metaphor for the light, a faucet as a metaphor for an aperture, and water pressure as a metaphor for a light condition. So, it likes we need to open a faucet for some period of time so that we can have enough water. The next question is how much is enough? The answer is that it directly depends upon the ISO sensitivity.
The ISO can be thought as a water bucket. The low ISO means the big bucket. More specifically, the lower ISO you use, the more water or light you need.
Next, the faucet or the aperture control how fast water or light can pass through to the water bucket or the camera's image sensor (or film). The wider you open them, the more amounts of water and light can reach the bucket or the sensor.
The water pressure determines the amount of water entering the faucet per a unit of time. Likewise, the light condition indicates the amount of light which is ready to go through the aperture. For example, in daylight at noon, there should be more ready amount of light than in the night. Below picture depicts that there is a huge pump during daylight so the water pressure must be extremely high. On the other hands, in the night, we do not have any pump, or we barely have source of light. That's why we generally need to turn a flash on.
(pictures source: go_med @ rpst-digital.org)
See also
ISO, Aperture, and Light Condition part 2/2
Learn about Shutter Speed and why it's so important
Aperture and F-number
More on Shooting Tips
