Enrico Marega Interactive photography course
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The exposure triangle

Three controls decide how much light ends up in the photo: aperture, shutter speed and ISO. Each one also has a side effect, and that’s where creative choices come from.

1Simulator

Expose the photo

Turn the three dials until the light meter reads zero. Then find the same light with different combinations and see what changes in the background, in the running person and in the grain.

What a stop is

1Simulator A stop is the step we use to count light, and it’s a doubling: +1 stop means twice the light, −1 stop half. Two stops are four times as much (2 × 2), three stops eight times (2 × 2 × 2). We think in doublings because the eye works that way too: going from one light bulb to two is as noticeable as going from a hundred to two hundred.

1Simulator Each control has its own stops. Shutter speed: from 1/250 to 1/125 the sensor is exposed for twice as long, +1 stop. ISO: from 400 to 800 the signal is amplified twice as much, +1 stop. Aperture: from f/4 to f/2.8 the opening has twice the area, +1 stop; the numbers change by about 1.4 times rather than 2 because what counts is the area of the opening, not its diameter.

1Simulator In the simulator each click of the dials is exactly one stop. That’s why you can swap one stop of aperture for one of shutter speed and the photo stays just as bright: f/2.8 at 1/250 gives the same exposure as f/4 at 1/125. On real cameras the dials usually move in thirds of a stop: three clicks make one stop.

How it works

1Simulator The aperture is the opening in the lens: small number, big opening. The shutter speed is how long the shutter stays open. ISO is like the volume knob on an amplifier: it boosts the signal, but along with the signal it boosts the hiss too, that is, the noise.

Key points

  • One stop = twice or half the light. One stop of shutter speed, one of aperture and one of ISO are equivalent.
  • Wide aperture (f/1.8) = more light and a blurred background. Narrow aperture (f/16) = less light and more in focus.
  • Fast shutter speed (1/1000) = motion frozen. Slow shutter speed (1/15) = motion blurred and, handheld, the whole photo shaken.
  • High ISO = a brighter photo but more grain. Raise it after choosing aperture and shutter speed.

Try it yourself

Photograph the same subject three times with the same exposure: once with a wide aperture, once with a narrow aperture, once with a slow shutter speed. An Android smartphone in Pro mode works too, or an iPhone with an app that lets you set shutter speed and ISO.

Post the result and tag me @enrico.marega, or send it to me by DM: I’d love to see how you tackled it.

Quick check

You go from f/4 to f/2.8. To keep the same exposure, how do you change the shutter speed?
Indoors, with a moving person, the photo is too dark at f/2.8 and 1/250. What do you increase?