How do shutter speed, ISO and aperture work together to control exposure?

Question

When shooting in natural or ambient artificial light (without flash, basically), how do light, shutter speed, ISO and aperture work together to control exposure?

Answer 1

To create a sort of analogy, let's consider a final, perfectly exposed photograph to be 100 litres of collected water, our camera, is the rainforest, and our camera's sensor, is a bunch of small buckets. We're going to play god here so we can control the environment (our camera) manually, and try to collect the rain using our buckets.

Now, we have several variables here with respect to how we will collect 100 litres of rainwater. It depends how heavily it is raining, how much rain will get down past the rainforest canopy, how long it is raining, and how many buckets we place on the ground. These are all analogous to parts of our camera.

How heavy the rain falls is analogous to how bright the scene you want to photograph is.

How long the rain falls is analogous to the shutter speed.

How many trees block the rain from reaching the forest floor is analogous to aperture.

How many buckets we place on the ground is analogous to the camera's ISO.

So let's consider placing 100 buckets on the forest floor (ISO 100), having a very dense canopy (f/16), and very light rain (a very dark scene). For us to collect that 100 litres of water, we would need to wait a very long time. For the purposes of this analogy, let's say 2 hours (a very long shutter speed in a dark scene). Now if we want to reduce the amount of time it takes to collect the water (because who has 2 hours to spare?), we can do a few different things:

1. Add more buckets to the forest floor (increase the ISO)
2. Remove some of the tree branches from the forest canopy (increase the aperture size, or lower the f-stop)
3. Play God, and make it rain harder (introduce more light to the scene)

Now, if we choose one of these options and try to collect that 100 litres again, we can be done in maybe 1 hour instead of 2, or if we combine all three of these options, it could be finished in as little as 15 minutes. The trick is striking the balance that is right for what you want and what the scene needs.

Now suppose that when we tried collecting our 100 litres of water in a forest with a very thin canopy, 6400 little buckets on the ground, and a torrential downpour, we'd have our 100 litres nearly instantly (overexposure)! To fix this, we have a few options again:

1. Add some tree cover to the canopy (decrease the aperture size, or increase the f-stop)
2. Remove some of the buckets from the ground (lower the ISO)
3. Play God and make it less rainy (using a neutral density filter)

The end goal is to balance these variables in a favourable way to create the perfect exposure - that 100 litres we are aiming for. Too much water (light), and our "photo" will be overexposed. Too little, and it will be underexposed. Once you understand this, you can manipulate your cameras settings to achieve other effects with depth of field, motion blur, and more!

I'd suggest taking many shots of the same scene with consistent light until you understand the effects of each setting, and what will happen when you raise or lower one of them!

Answer 2

Light is either directly coming from its source or is being reflected by some object into the opening of your lens.

The aperture controls how big that opening of the lens is.

The shutter speed determines how long that opening is open. More precisely: for how long light going through the lens can reach the sensor.

ISO determines how much light will result in what brightness in the final image.

very basically speaking.