How do aperture, shutter speed and ISO trade light against blur and noise?

A photograph is a tiny harvest of light, and you get three dials to control how much you collect. The aperture is the size of the hole in the lens, the shutter speed is how long that hole stays open, and ISO is how much the camera amplifies what it gathered. Exposure, the light that reaches the film or sensor, is set by the time and the opening, together with how bright the scene is.

The surprise is that the dials trade against each other. The size of the opening is written as an f-number, and each "stop" (about 1.4 times in f-number) halves or doubles the light. f/8 lets in four times as much light as f/16. The shutter works on the same doubling logic, so 1/50 s at f/4 gives the same exposure as 1/100 s at f/2.8, or 1/200 s at f/2.

So why not always pick the same pair? Because each dial has a side effect. A wide opening shrinks depth of field, so only a thin slice of the world is sharp. A slow shutter risks motion blur. And a high ISO usually brings coarser grain or more digital noise. Choosing an exposure is choosing which of these three costs you can live with.

Stanley Kubrick lived with an extreme one. To film candlelit scenes of Barry Lyndon, he used a lens with an f/0.7 opening, the fastest in film history, and accepted that very little of each scene could be in focus at once.