The Exposure Triangle

1 · The Idea (the concept in plain language)

Every photograph is made of light collected over a moment of time. "Exposure" is just the word for how much light lands on the sensor — and therefore how bright or dark the final image looks. Too much light and the photo is overexposed (washed-out, blown-out whites with no detail). Too little and it's underexposed (muddy, crushed shadows). Somewhere in between is the brightness you want.

You have exactly three controls over that amount of light, and that's the whole game:

• Aperture — how wide the hole in the lens opens. A wider hole lets in more light. (Written as an f-number: f/2.8, f/5.6, f/11 …)
• Shutter speed — how long the sensor is exposed. A longer time lets in more light. (Written as a fraction of a second: 1/1000s, 1/60s, 1s …)
• ISO — how much the camera amplifies the light it caught. A higher ISO makes a dim signal brighter after the fact. (Written as a plain number: 100, 400, 3200 …)

That's the famous "exposure triangle." It's a triangle because the three sit in balance: brighten one and the picture gets brighter, so you usually have to darken another to keep the brightness where you want it. They trade against each other.

Here's the part that makes photography an art instead of a math problem: each of the three changes the look of the photo in a completely different way, on top of changing brightness. So they're not interchangeable. Choosing which control to move — that's you making a creative decision, whether you realize it or not.

2 · What Your Camera Is Doing (the mechanism, jargon-light)

To trade the three controls fairly, you need one shared unit of "amount of light." That unit is the stop.

A stop = doubling or halving the light. That's the entire definition. One stop brighter means twice as much light reached the sensor; one stop darker means half as much. And — this is the beautiful part — all three controls move in stops, so a stop of aperture, a stop of shutter, and a stop of ISO are all the same size. That's what lets them trade one-for-one.

Here's how each control makes a stop:

• Shutter — easy, it's just time. Double the time, double the light. 1/125s → 1/60s is one stop brighter (the shutter is open roughly twice as long). 1/125s → 1/250s is one stop darker. The number sequence looks lumpy — 1/125, 1/60, 1/30 — because cameras round it off, but each really is a doubling or halving of duration.
• Aperture — a little sneaky, because it's an area. The f-number is a fraction (focal length ÷ opening width), so a bigger f-number means a smaller hole. To double the light you have to double the area of the opening, which is why the full-stop f-numbers jump in an odd-looking pattern — f/2.8 → f/2 → f/1.4 each doubles the light. The trick to remember: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 — moving one name along this list is exactly one stop.
• ISO — pure amplification. ISO 200 → 400 is one stop brighter; the camera turns up the "volume" on the signal by 2×. It doesn't gather any new light — it just amplifies what the shutter and aperture already let in, which is why cranking it eventually amplifies the graininess (noise) too.

The camera measures the scene with its built-in light meter, which looks at the brightness coming in and calculates what combination of the three will produce a "correct" exposure. But "correct" to a meter has a very specific, slightly dumb meaning — we'll unpack it in §4. For now, hold onto this: the meter tells you when a combination is balanced; it doesn't tell you which balanced combination to pick. That choice is yours.

3 · The Controls (where this lives on the camera)

Every ladder below is in full stops — each step is a doubling or halving of light. Real cameras usually click in ⅓-stop increments (three clicks per full stop), which is why you'll see in-between values like f/3.5, 1/200s, or ISO 250. Those are just finer steps on the same ladders.

The three ladders (each step = 1 stop of light):

Each lever's side effect (the reason you'd pick one over another):

What each shooting mode hands to you vs. keeps for itself:

Settings notation used throughout: every concrete setting is written Mode f/x · 1/xs · ISO x — e.g. a safe daylight starting point is A f/5.6 · 1/250s · ISO 200. When the camera is choosing a value for you, we'll write it as (auto).

4 · In Practice (how to actually use it)

Equivalent exposures — the key move

Because all three levers speak in stops, the same brightness can be reached many different ways. Give aperture +1 stop and shutter −1 stop and you've changed nothing about brightness — but you have changed the depth of field and how motion renders. Photographers call these equivalent exposures: same brightness, different picture.

Here's a ladder of five settings that all produce exactly the same brightness. Each step closes the aperture one stop (halving the light) and lengthens the shutter one stop (doubling it) — the two cancel perfectly:

All five are the same photo in brightness. They are wildly different photos in look. That difference is where you make your creative choice — and it's why "what settings should I use?" never has one answer.

A worked example — choosing by side effect

You're indoors photographing a friend who won't hold still. You're in Aperture-priority on your 50mm f/1.8 prime, and at A f/4 · (auto) · ISO 800 the camera picks 1/60s. Full setting: A f/4 · 1/60s · ISO 800. The brightness is fine — but 1/60s is too slow to freeze a fidgeting person, so they come out slightly soft.

You decide you need 1/250s to freeze them, so you switch to M to set it yourself. Going from 1/60s to 1/250s is 2 stops faster shutter — that's 2 stops less light. You have to put those 2 stops back somewhere, and you have two honest choices:

• Put it back with aperture: open from f/4 to f/2 (2 stops wider). Result: M f/2 · 1/250s · ISO 800 — same brightness, motion frozen, but a much shallower depth of field (you're trading away depth of field to buy shutter speed).
• Put it back with ISO: raise from ISO 800 to ISO 3200 (2 stops). Result: M f/4 · 1/250s · ISO 3200 — same brightness, motion frozen, depth of field unchanged, but a noisier image (you traded away cleanliness instead).

Neither is "correct." You're choosing which side effect you can live with in this scene. That single decision — depth of field vs. noise vs. motion — is the entire skill of exposure.

A pocket cheat-sheet: goal → mode → starting point

The meter, and why "correct" is a creative choice

Your camera's meter aims to make the scene come out to an average middle brightness (a medium gray). That's a sensible guess most of the time — and a terrible one when the scene is mostly very light or very dark, because the meter doesn't know the difference between "a bright scene" and "an over-lit ordinary scene." So it gets fooled:

• Snow, a white wall, a bride's dress → the meter sees "too bright," darkens everything, and your snow comes out dingy gray. Fix: dial in +1 to +2 exposure compensation to tell it "no, this really is bright."
• A black cat, a night street, a dark suit → the meter sees "too dark," brightens everything to gray, and washes the mood out. Fix: dial in −1 to −2 exposure compensation.

Exposure compensation (the ± button/dial) nudges the meter's target brighter or darker in P, A, and S — it's how you overrule the meter without leaving the semi-auto modes. (In M it does nothing on its own, because you're already setting everything by hand — unless Auto ISO is on, in which case it shifts the ISO the camera picks.)

The deeper point: there is no single "correct" exposure — only the brightness you intend. A high-key, airy portrait is deliberately brighter than the meter wants; a moody, low-key still life is deliberately darker. The meter gives you a starting point; you decide the final brightness, and the histogram (the little graph of tones) is your honest referee — watch that the highlights you care about aren't jammed against the right edge (blown out) or the shadows against the left (crushed).

5 · Feel It Yourself (hands-on — this is where it clicks)

Every exercise starts with a count and a constraint, names the one thing to vary, and says what success looks like. You can do most of these at a table with a couple of objects and a window.

1. Take 5 shots of one still object using the equivalent-exposure ladder. Put a mug in front of a cluttered shelf, go to M, fix ISO 400, and shoot the ladder from §4: f/2.8 · 1/1000s → f/4 · 1/500s → f/5.6 · 1/250s → f/8 · 1/125s → f/11 · 1/60s. (On the kit lens, which can't open past ~f/5.6, run this reachable ladder instead: f/5.6 · 1/250s → f/8 · 1/125s → f/11 · 1/60s → f/16 · 1/30s → f/22 · 1/15s — each step still closes the aperture 1 stop and lengthens the shutter 1 stop, so it shows the same effect.) Vary only that aperture/shutter pairing. Success: all 5 frames are the same brightness, but the shelf behind the mug goes from a soft blur to sharp clutter as you climb. That's equivalent exposure — one brightness, five looks.

2. Take 5 shots in Aperture-priority, opening the aperture one stop at a time. Frame anything, switch to A, keep ISO fixed, and step f/11 → f/8 → f/5.6 → f/4 → f/2.8 (start at f/5.6 if you're on the kit lens). Vary only the aperture, and watch the shutter speed the camera reports each time. Success: you can predict it — every 1 stop you open the aperture, the camera shortens the shutter by 1 stop to compensate. You've just watched the triangle balance itself.

3. Take 5 shots in Shutter-priority of something moving (a running tap, a fan, a friend waving). Switch to S + Auto ISO and step 1/1000s → 1/250s → 1/60s → 1/15s → 1/4s. Vary only the shutter. Success: the fast frames freeze the motion crisp, the slow frames streak it — and you can see the aperture (and ISO) the camera moved to keep brightness steady.

4. Take 5 shots of the same dim indoor scene, raising ISO one stop at a time. In M, lock f/2.8 · 1/125s (or your lens's widest at 1/125s) and step ISO 400 → 800 → 1600 → 3200 → 6400. Vary only the ISO. Success: brightness climbs one stop each frame, and when you zoom to 100% you can see the grain appear — the ISO where it starts to bother you is your personal ISO ceiling. Now you know it.

5. Take 5 shots of a bright white or snowy subject using exposure compensation. A white plate on a white cloth, or snow, in A. Step exposure compensation −1 → 0 → +1 → +1.7 → +2. Vary only the ± dial. Success: at 0 the white looks muddy gray (the meter fooled itself); somewhere around +1 to +1.7 the white finally looks white. You've just met the meter's bias in person.

6. (Optional stretch) Take 5 shots reaching one target exposure three different ways. Pick a scene, note what the meter suggests, then in M hit that same brightness once via aperture, once via shutter, once via ISO. Vary which lever does the work. Success: all three match the meter's brightness, but differ in depth of field, motion, and noise — proof that "correct exposure" is a family of choices, not a single answer.

6 · Common Confusions (what trips everyone up)

7 · Recap

• Exposure is one thing — brightness — set by three controls: aperture (the hole), shutter (the time), and ISO (the amplification). They work as one linked system, not three separate problems.
• A "stop" is a doubling or halving of light, and all three controls speak in stops — so +1 stop on one lever and −1 stop on another leaves brightness untouched.
• Equivalent exposures: the same brightness is reachable many ways. You choose the combination by its side effect — aperture → depth of field, shutter → motion, ISO → noise.
• Let the camera drive one lever while you own the creative one: A / Av when depth of field matters, S / Tv when motion matters, both with Auto ISO; go Manual when you want the same exposure every frame or the meter keeps getting fooled.
• The meter's "correct" is only an average-gray starting point. Overrule it with exposure compensation (+1 to +2 for bright/snowy scenes, −1 to −2 for dark ones) and trust the histogram.
• Gear reality: the kit lens (f/3.5–5.6) pushes you toward the ISO lever in dim light — that's fine. The 50mm f/1.8 nifty fifty buys you ~2–3 extra stops of freedom to spend on shutter or ISO instead.
• The confusions you now beat: big f-number = small hole; "fast lens" = wide aperture; "fast shutter" = short time.