Shutter speed is how long light is collected — it decides motion blur. Aperture is how wide the opening is — it decides depth of field. ISO is how much the signal is amplified — it decides noise.
All three are measured in stops, and a stop is a doubling or halving of light. That shared unit is the whole trick: give up one stop on one, take one stop back on another, and the brightness is unchanged.
First, what a stop is
Everything else depends on this one idea, and it's simple: a stop is a doubling or halving of light.
One stop brighter can come from any of three places:
- Doubling the shutter time — 1/125 becomes 1/60
- Opening the aperture one full step — f/8 becomes f/5.6
- Doubling the ISO — 400 becomes 800
All three do the identical thing to brightness. That's why they're tradeable, and why photographers talk in stops rather than in the individual numbers.
The aperture numbers look irrational until you know they're a square-root sequence — 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 — because a circular opening's area is what matters, and area scales with the square of the radius. Every step in that sequence halves the light. You don't need the maths; you need to recognise the sequence.
The three settings
Shutter speed — the motion dial
How long the sensor is exposed, in fractions of a second. Fast freezes, slow blurs.
The side effect is the point. 1/1000 stops a splash mid-air. 1/30 turns a walking person into a smear. Neither is wrong — a waterfall at 1/1000 looks like frozen gravel, and at 2 seconds it looks like silk. You're choosing how time appears in a still image.
Aperture — the depth dial
How wide the opening in the lens is, written as an f-number. Confusingly, smaller number means wider opening: f/1.4 is wide, f/16 is narrow.
Wide apertures let in more light and produce shallow depth of field — the blurred background in portraits. Narrow apertures let in less light and hold more of the scene sharp, which is why landscapes are shot at f/8 or f/11.
ISO — the amplification dial
How much the signal off the sensor is amplified. Higher ISO means a brighter image from the same light, and more visible noise, because the noise gets amplified too.
ISO is the one to move last, but not the one to avoid. An underexposed low-ISO file that you brighten later looks worse than a correctly exposed high-ISO one, because you amplify the noise in editing anyway.
Why it's a triangle
Picture the same scene, correctly exposed three different ways:
| Shutter | Aperture | ISO | The photo you get | |
|---|---|---|---|---|
| A | 1/500 | f/2.8 | 400 | Motion frozen, background blurred |
| B | 1/125 | f/5.6 | 400 | Some blur, more depth in focus |
| C | 1/125 | f/2.8 | 100 | As A's depth, but motion softer |
All three are the same brightness. They are completely different photographs. Row A to row B gives up two stops of shutter and takes two back on aperture — identical exposure, and now the background is sharp.
That's the whole concept. Exposure isn't the interesting part; it's the constraint. What you're really choosing is which side effect you want.
Want to see the trade numerically? The equivalent exposure calculator rebalances the other two as you change one.
Which to set first
The question that unlocks it: what would ruin this photograph?
- Motion would ruin it — a running child, a bird, a handheld long lens. Set shutter speed first.
- Wrong depth would ruin it — a portrait needing a soft background, a landscape needing front-to-back sharpness. Set aperture first.
- Noise would ruin it — a night sky, a long exposure, something you'll print large. Set ISO first.
Set that one, let the camera solve the others, and adjust from there. This is exactly what the priority modes on a real camera do — Shutter priority and Aperture priority aren't training wheels, they're the fastest way to work when light is changing.
Going full manual too early. Full manual is right when light is constant — a studio, a tripod, a locked-off scene. Handheld outdoors, the light changes every time you turn around, and you'll spend the whole shoot re-metering while the moment leaves.
Cheat sheet
Starting points. Meter, then adjust.
| You're shooting | Hold this | Start at |
|---|---|---|
| Portrait, blurred background | Aperture | f/1.8–f/2.8, 1/250, ISO 100 |
| Landscape, sharp throughout | Aperture | f/8–f/11, ISO 100, shutter floats |
| Sport / kids / pets | Shutter | 1/500 or faster, ISO floats |
| Street, handheld | Shutter | 1/125, f/8, ISO floats |
| Indoors, low light | Shutter | 1/60 minimum, widest aperture |
| Waterfall, silky | Shutter | 1/2–2s, tripod, ISO 100, add ND |
| Night sky | ISO | ISO 1600–3200, 15–25s, widest aperture |
| Concert | Shutter | 1/250, widest aperture, ISO floats high |
And the meterless fallback, worth memorising: Sunny 16. In bright sun, set f/16 and shutter to one over your ISO — ISO 100 means 1/100. Open one stop for hazy sun, two for cloudy, three for overcast, four for open shade.
On a phone, it's a line
Phone lenses have a fixed physical aperture. There's no diaphragm to open or close — an iPhone main camera sits permanently at around f/1.8. The triangle loses a leg and becomes a straight trade between shutter speed and ISO.
That's simpler, not worse. Two variables instead of three, and the decision is always the same one: how much motion blur will you accept, and how much noise.
It also means any phone app showing an aperture slider is showing you something that doesn't exist optically. At best it's simulating background blur afterwards — a legitimate effect, but not aperture, and it teaches you something false about how cameras work.
Depth of field on a phone is controlled by distance: get closer to your subject, and put more space behind them.
Common questions
What is the exposure triangle?
The relationship between shutter speed, aperture and ISO — the three settings that decide brightness. They're linked, so changing one means changing another to hold the same exposure, and each carries a side effect: motion blur, depth of field, and noise respectively.
What's a stop?
A doubling or halving of light. One stop can come from doubling the shutter time, opening the aperture one full step, or doubling the ISO. The shared unit is what makes the three tradeable.
Is ISO really part of exposure?
Strictly, no — exposure is the light reaching the sensor, which only shutter and aperture control. ISO amplifies afterwards. In practice it trades exactly like a stop of light, so it belongs in the arithmetic even if it doesn't belong in the physics.
Why do smaller f-numbers mean more light?
Because the f-number is a ratio — focal length divided by the diameter of the opening. A bigger opening produces a smaller ratio. f/2 on a 50 mm lens means a 25 mm opening; f/8 means about 6 mm.
Should I shoot in full manual?
When the light is constant, yes. When it's changing, priority modes are faster and more accurate — hold the setting that matters and let the camera solve the rest. Professionals use priority modes constantly; it isn't a shortcut.
Keep going
Hold one leg. Let the camera float the rest.
Lucid puts priority modes on your iPhone — pick the setting you're protecting and it solves the others live, exactly like a real camera body.