The Exposure Triangle, Explained
Aperture, shutter speed, and ISO are the three controls that determine every exposure. Each one affects how much light reaches the sensor — and each one carries a side effect you can't avoid. Understanding those trade-offs is what separates intentional photography from guesswork.
The three controls
The "triangle" is a teaching metaphor: three independent settings that together determine whether an image is correctly exposed. Change any one of them and you change the exposure — but you also change something else about the image.
Aperture
The aperture is the opening inside the lens. A wider opening admits more light; a narrower one admits less. Aperture is described as an f-number — f/1.8, f/4, f/11 — and the scale runs backward: a smaller f-number means a larger physical opening. (The f-number is a ratio of focal length to opening diameter, so f/1.4 on a 50 mm lens is a 35 mm opening — much larger than f/16 at 3 mm.)
The side effect of aperture is depth of field. A wide aperture (low f-number) blurs the background, isolating a subject. A narrow aperture (high f-number) keeps near and far objects sharp together. You can explore how focal length and focus distance interact with aperture using the depth-of-field calculator.
Shutter speed
Shutter speed is how long the sensor is exposed to light — typically expressed as a fraction of a second: 1/500 s, 1/60 s, 1/4 s. A faster shutter admits less total light; a slower one lets in more.
The side effect of shutter speed is motion. A fast shutter freezes a moving subject sharply. A slow shutter allows a subject — or camera shake — to trace a blur across the frame. For most handheld shooting, the old rule of thumb is to keep the shutter speed denominator at or above the focal length (shoot a 50 mm lens at 1/60 s or faster). Whether that blur is a problem or an artistic choice is entirely up to you.
ISO
ISO sets the amplification applied to the signal the sensor captures. At ISO 100 the signal is used as-is; at ISO 1600 it is amplified roughly four stops' worth. Higher ISO lets you shoot in dim conditions without slowing the shutter or widening the aperture.
It is important to be precise here: ISO does not change the amount of light the lens gathers. The lens and aperture determine how many photons hit the sensor during the exposure. ISO amplifies the resulting electrical signal after the fact. The side effect is noise — the random variation in that amplified signal that appears as grain, color speckle, and lost shadow detail. Modern sensors handle ISO 3200 cleanly; ISO 12800 and above will show visible noise on most cameras.
What a "stop" means
A stop is the universal unit of light in photography: one stop = doubling or halving the amount of light reaching the sensor. The term comes from aperture rings on older lenses that had physical detents ("stops"), but it now applies to all three controls.
- Aperture stops: f/1.4 → f/2 → f/2.8 → f/4 → f/5.6 → f/8 → f/11 → f/16. Each step right halves the light (the area of the opening changes by a factor of 2 because the area scales with the square of the diameter).
- Shutter speed stops: 1/500 → 1/250 → 1/125 → 1/60 → 1/30. Each step left doubles the exposure time, doubling the light.
- ISO stops: 100 → 200 → 400 → 800 → 1600. Each doubling of ISO amplifies the signal by one stop.
Most cameras also offer third-stop or half-stop increments between these round values, which is why you'll see f/3.5, ISO 640, and 1/320 s on modern menus.
Equivalent exposures
Because all three controls are measured in stops, they are directly interchangeable for the purpose of brightness: give up a stop in one setting and you can recover it in another. This is the practical payoff of understanding the triangle.
Suppose you are shooting at f/4, 1/250 s, ISO 400 and decide you want a shallower depth of field — so you open the aperture to f/2, which is two stops more light. To keep the same brightness, you need to remove two stops elsewhere. You could increase the shutter speed two stops to 1/1000 s, or drop ISO two stops to ISO 100, or split the difference. The subject's apparent brightness stays the same; only the depth of field and the motion rendering change.
Exposure Value (EV)
Exposure Value is a single number that summarizes an aperture-and-shutter combination at a standard sensitivity. The formula is:
EV = log₂(N² ÷ t)
where N is the f-number and t is the shutter speed in seconds. EV is defined at ISO 100 — at other ISOs the effective EV shifts by the number of stops the ISO is away from 100. EV 0 corresponds to f/1, 1 s at ISO 100; EV 15 is roughly the brightness of a sunlit scene (the basis of the sunny 16 rule, described below). Because EV collapses aperture and shutter into one number, it is useful for metering and for discussing light levels independently of how you choose to expose them. The exposure value calculator will compute EV for any aperture-shutter pair and show you all the equivalent combinations.
Metering and exposure compensation
A camera's meter measures the light reflected from the scene and proposes an aperture-shutter-ISO combination it thinks will produce a mid-tone (18% grey) average. Most of the time this is a useful starting point. Where it fails is when the scene is dominated by a very bright area (snow, sky) — the meter tries to darken it toward grey — or a very dark area, which it tries to lighten.
Exposure compensation (the +/- button) lets you tell the camera to expose brighter or darker than the meter suggests, in stop increments. Shooting a white snow scene? Try +1 to +2 stops. Shooting a dark subject against a bright sky? Dial in −1 to see if the sky retains detail. The number is added to (or subtracted from) whatever exposure the meter proposes.
Practical starting points
Sunny 16
In direct sunlight, set aperture to f/16 and shutter speed to the reciprocal of your ISO. At ISO 100 that means 1/100 s (or the nearest available value, 1/125 s). At ISO 400 use 1/400 s (or 1/500 s). This rule is reliably within a stop of correct in bright sun and is a useful mental reset when metering behaves oddly.
Freezing motion
Walking subjects freeze at around 1/250 s. Sports and running subjects typically need 1/500 s to 1/1000 s. Birds in flight or fast action may require 1/2000 s or faster. Once you commit to a shutter speed, you buy back the lost light through a wider aperture or higher ISO — the triangle in action.
Low light and night sky
In very low light the triangle's limits become visible: you've already opened the aperture to its maximum and raised ISO to the point noise is a concern, yet the shutter is still too fast to collect enough light. The solution is a longer exposure — which reintroduces motion, whether from a subject or from the apparent movement of stars. For night-sky shooting, the 500 rule caps shutter speed to avoid star trails: divide 500 by the focal length to get the maximum seconds before stars begin to streak. The night sky 500 rule calculator applies this for any focal length and sensor crop factor.
Putting it together
The exposure triangle is not really a formula — it is a way of thinking. Every time you adjust one control you are making a trade: light for depth, sharpness for noise, motion for brightness. A good exposure is the one where the trade you made was the one you actually wanted. Once those trade-offs are instinctive, the camera's controls become expressive tools rather than technical obstacles.