Exposure, the whole picture
The three levers — aperture, shutter, ISO — measured in stops, and why every exposure is a trade.
- Define a stop and count exposure changes in stops
- Explain how aperture, shutter, and ISO trade against each other
- Use exposure compensation to place brightness where you want it
Exposure is how much light ends up in the picture, and it is set by three controls pulling against each other. Master the trade between them and you can make any brightness you want, on purpose.
Stops: the universal unit
A stop is a doubling or halving of light. Open the aperture one stop and twice as much light comes in; use a shutter speed one stop faster and half as much does. Raise ISO a stop and the signal is amplified twofold. Because all three speak in the same unit, you can trade one for another exactly: give up a stop here, take it back there, and the brightness holds.
The full-stop scales are worth knowing by heart:
- Aperture: f/2.8 · f/4 · f/5.6 · f/8 · f/11 · f/16
- Shutter: 1/30 · 1/60 · 1/125 · 1/250 · 1/500 · 1/1000
- ISO: 100 · 200 · 400 · 800 · 1600 · 3200
The three levers
- Aperture controls light and depth of field (Lesson 9).
- Shutter controls light and how motion renders (Lesson 10).
- ISO controls brightness and noise (Lesson 11).
Because each lever has a side effect, exposure is never just about brightness — it is about which side effects you want. That is the whole game.
Move one lever a stop and another must move back a stop to hold the same exposure.
Move one lever in the demo and watch the meter swing; move another back to re-centre it. That is a photographer’s constant small arithmetic: hold the exposure, choose the trade.
Exposure compensation
In A mode the camera picks the shutter for a “correct” middle exposure — but “correct” is an average, and averages get fooled by snow and by spotlights. Exposure compensation is your override: dial it plus to brighten, minus to darken, and the camera keeps your intent while still solving the rest. It is the fastest way to say “no, I meant it brighter.”
In the field
Put the camera in A mode at a fixed aperture. Photograph a white wall, then a dark one, letting the camera decide. Both come out grey. Now use exposure compensation to make the white wall white and the dark wall dark. You have just out-voted the meter — a skill you will use forever.