Understanding Exposure: Aperture, Shutter, ISO

Everything in photography eventually comes back to three controls, and they are connected like three taps filling the same bucket. Master how they trade against each other and you will never be afraid of a tricky light situation again. This is the exposure triangle, and the simulator on this page lets you feel it rather than just read about it.

The bucket of light

A correct exposure is simply the right amount of light reaching the sensor: not so little that the shadows go black and empty, not so much that the highlights blow out to featureless white. Three controls govern how much light arrives and how it looks.

Aperture: the size of the opening

Aperture is the hole in the lens, measured in f-stops. Confusingly, smaller numbers mean a bigger hole. f/2.8 is wide open and lets in a lot of light; f/16 is a small opening and lets in little. Each full stop, f/2.8 to f/4 to f/5.6 to f/8 to f/11 to f/16, halves the light.

Aperture also controls depth of field, the slice of the scene that is in focus. Wide open at f/2.8, only a thin plane is sharp and the background melts. Stopped down to f/8 or f/11, much more is sharp front to back. This is why street photographers live at f/8: it gives enough depth of field to keep a moving scene sharp without thinking.

Shutter speed: how long the door stays open

Shutter speed is the length of time the sensor is exposed, from 1/4000 of a second up to a full second or longer. Each halving or doubling is one stop: 1/500 lets in half as much light as 1/250.

Shutter speed controls motion. Fast shutters, 1/500 and up, freeze a walking person or a passing cyclist. Slow shutters, 1/15 and below, let motion blur into streaks, which can be a mistake or a deliberate, beautiful effect. Remember: the in-body stabilization on the GR IV fixes your shake, not your subject's movement. A person walking will blur at 1/30 no matter how steady your hands are.

ISO: the sensitivity dial

ISO is how strongly the sensor amplifies the light it gathers. ISO 100 is the cleanest, lowest sensitivity, for bright light. As you raise it, 400, 1600, 6400 and up, you can shoot in darker conditions, but you introduce grain, the fine speckled texture. On the GR IV the grain is well controlled and, in black and white, often reads as character rather than noise. Still, use the lowest ISO the light allows.

Reciprocity: the trade that keeps exposure constant

Here is the key idea. Because each control works in stops, you can trade one against another and keep the same exposure. f/4 at 1/500 and ISO 200 lets in exactly as much light as f/8 at 1/125 and ISO 200, or f/8 at 1/250 and ISO 400. These are equivalent exposures. They are equally bright but they look different: the f/8 version has more depth of field, the 1/500 version freezes more motion. Photography is choosing which equivalent exposure tells your story.

Sunny 16, the rule that frees you from the meter

On a bright sunny day, set the aperture to f/16 and the shutter speed to one over your ISO. At ISO 100 that is 1/100, near enough 1/125. That is a correct daylight exposure with no meter at all. Open up the aperture and speed up the shutter together as the light dims. Sunny 16 is worth memorizing because it teaches you to read light with your eyes, which is what separates photographers from people who own cameras.

Now go feel it

Open the Exposure Simulator below. Drag the aperture slider and watch the background blur and the brightness change together. Drag the shutter and watch the walking figure freeze or smear. Drag the ISO and watch the grain creep in. Then try the five Bologna challenge scenarios. The numbers will become instinct faster than you expect.