Exposure Triangle: Aperture, Shutter Speed, and ISO Together

What the exposure triangle is

The exposure triangle is a way to visualize how the three camera controls aperture, shutter speed and ISO interact to determine how bright or dark a photo is. If you change any one of them without adjusting another, the brightness of your picture changes. Raise the shutter speed by one stop (for example, from 1/125 to 1/250 second) and the sensor is exposed to half as much light. Open the aperture by one stop (from f/8 to f/5.6) and twice as much light reaches the sensor. Reduce the ISO from 400 to 200 and the sensor becomes half as sensitive.

In practice, you rarely adjust one control alone. Instead, you set the three together so that the sensor receives the amount of light you want while you also balance each control's side effects. The two most important side effects are depth of field (controlled by the aperture) and motion blur or camera shake (controlled by the shutter speed). The third, image noise (controlled by the ISO setting), becomes more visible as sensitivity goes up.

Many cameras offer semi-automatic modes that use the same idea. Aperture-priority mode lets you set the lens opening while the camera chooses a shutter speed. Shutter-priority mode does the opposite. Full manual mode gives you both, plus the ISO. Understanding the triangle helps you predict what any mode will do.

Aperture: light and depth of field

The aperture is the opening inside the lens that lets light through. A wider opening (a lower f-number) lets in more light, and a narrower opening (a higher f-number) lets in less. A one-stop aperture change roughly doubles or halves the light that reaches the sensor.

At the same time, the aperture changes depth of field, the zone of acceptable sharpness in the image. At a wide opening like f/1.8, the zone of sharp focus is shallow, and a portrait subject can be separated from a busy background. At a narrow opening like f/11, much more of the scene stays acceptably sharp, which is useful for landscapes and architecture.

If you want the background blur that wide apertures give, you may set the aperture first and let the shutter speed fall where it may. That approach is exactly what aperture-priority mode automates.

For a closer look at how the lens opening affects exposure and creative results, see the detailed explainer on aperture.

Shutter speed: light and motion

The shutter speed is how long the sensor is exposed to light. A longer shutter speed collects more light, which helps in dim conditions, but it also records more motion blur: a moving subject can become a streak, and a handheld camera can show shake. A shorter shutter speed freezes action but lets in less light.

The right shutter speed for a shot depends on what you want to convey. A fast shutter speed around 1/1000 second can freeze a bird mid-flap. A slow shutter speed of a few seconds can turn a waterfall into a silky blur, but then a tripod is necessary to keep the static parts sharp. The shutter speed also interacts with image stabilization, which helps handheld shooting at slower speeds, but there are limits.

When your subject moves fast, or when you are shooting handheld in dim light, the shutter speed is usually the most important setting to choose first. Set it high enough to avoid unwanted blur, then adjust the aperture and ISO to get the exposure.

The trade-off between action freezing and motion blur is covered in more depth in the shutter speed guide.

• Fast shutter speeds freeze motion; slow shutter speeds blur it by design.
• Camera shake becomes visible at handheld speeds roughly below a focal-length-based limit; image stabilization shifts that limit.
• When motion is your subject, decide first whether to freeze or blur it.

ISO: sensitivity and its cost

ISO is the sensor's sensitivity setting, and it has a price: higher ISO values amplify the signal, but they also make image noise more visible. At low ISO values like 100 or 200, image quality is at its best. At high values like 3200 or 6400, you gain the ability to shoot in dim light, but you may see more grain and less color accuracy.

Modern cameras have improved high-ISO performance considerably, but the basic trade-off remains. A larger sensor, all else being equal, tends to produce cleaner images at high ISO than a smaller sensor because each photosite captures more light. This is one reason sensor size matters when you often shoot in low light.

The rule of thumb that has served photographers for generations is: use the lowest ISO that still allows the combination of aperture and shutter speed you need for your shot. Start at the base ISO and raise it only when the other two controls cannot do the job alone.

For a more complete treatment of sensitivity, noise and dynamic range, read the ISO sensitivity guide.

How the three work together

Think of the amount of light reaching the sensor as a cup that needs a certain volume of water. The aperture is the width of the faucet, the shutter speed is how long the faucet stays open, and ISO changes the size of the cup. If you make the faucet narrower (stop down the aperture), you can keep the water volume the same by leaving it on longer (slower shutter) or by using a smaller cup (higher ISO) so that less water fills it the same way. Each adjustment has its side effect: a narrow aperture gives more depth of field, a slow shutter can blur motion, and a high ISO adds noise.

In exposure terms, a stop is a doubling or halving of light. If you close the aperture by one stop, you let in half the light. To keep the exposure the same, you can halve the shutter speed (from 1/500 to 1/250, for example) or double the ISO (from 400 to 800). Small adjustments of any two controls can cancel each other exactly; the combination is what determines the brightness of the final image.

The same idea applies whether you use an optical viewfinder or an electronic viewfinder. Interchangeable lens systems, such as Canon RF, Nikon Z, Sony E, and Micro Four Thirds, all give you the same three controls on the body, with the lens providing the aperture. The difference is in how far you can push each setting before the image quality suffers.

To see how the triangle interacts with the larger sensor system, compare the full-frame advice with mirrors on a Micro Four Thirds system.

How to prioritize the three settings

Start with the setting that matters most for your photograph. When the subject is a person and you want to isolate them from the background, choose the aperture first. When you are photographing a race or a flying bird, choose the shutter speed first to make sure the action is sharp. When the light is fading and you cannot get the exposure you need with the first two, raise the ISO as the last resort.

For landscape work, a narrow aperture such as f/8 or f/11 is commonly chosen to keep near and distant detail sharp, which often means the shutter speed ends up slow enough to need a tripod. For portraits, a wide aperture like f/1.8 or f/2.8 blurs the background, and the shutter speed often remains reasonably fast. For street photography, a medium aperture such as f/4 or f/5.6 and a moderate shutter speed offer a balance of sharpness and mobility.

If you are uncertain about which setting to start from, most cameras have a Program mode that lets you change the aperture-shutter combination while preserving the same exposure; you can then adjust for the look you want. The important thing is to understand what you are trading wherever you move the dial.

What different camera types mean for the triangle

The exposure triangle applies to every digital camera, but the range of possible settings differs between models. A mirrorless or DSLR with a large sensor such as a full-frame or APS-C generally offers a wider usable ISO range and more flexibility in how far you can push any corner of the triangle before image quality degrades. A 1-inch compact or a smartphone-style camera has a smaller sensor and a narrower useful ISO range, so you may need to rely more on a wide aperture and a slow shutter speed in dim light.

Lenses also set limits on the aperture. A prime lens with a wide maximum aperture such as f/1.8 gives you more light-gathering ability than a typical kit zoom that starts at f/3.5. If you often shoot in low light, the lens's maximum aperture matters as much as the camera body.

When you choose your next camera, think about which side of the triangle you tend to hit first. Dedicated sports shooters value fast shutter speeds and wide apertures. Portrait shooters care about aperture for depth of field. Low-light shooters need a sensor that stays clean at high ISO. That is why the buying guides on this site compare cameras by sensor size, lens mounts, and video specs: these specs roughly determine how far you can push each control.

• Full-frame and APS-C cameras generally allow higher ISOs with less noise.
• Wide-aperture lenses (f/1.8, f/1.4) collect more light before you must raise ISO.
• Small-sensor cameras like 1-inch compacts need more light or higher ISO to compensate.

What to pick for your use

Questions

Which of the three settings matters most for exposure?

None by itself. The combination of aperture, shutter speed and ISO determines brightness. What matters is which setting you prioritize for the look you want: aperture controls depth of field, shutter speed controls motion blur, and ISO controls noise.

Should I always shoot in manual mode?

Not necessarily. Aperture-priority mode is a good starting point: you set the aperture, and the camera picks a shutter speed. Manual gives you full control when you need consistency, such as when the light is constant and you have time to set each control.

What is a stop in photography?

A stop is a doubling or halving of the amount of light reaching the sensor. Changing the aperture, shutter speed or ISO by one stop halves or doubles the exposure or sensitivity. Two changes of one stop in opposite directions cancel each other out.

Why are my photos dark even though I set the ISO high?

A high ISO alone does not guarantee a good exposure. If the aperture is too narrow or the shutter speed too fast, the sensor may still not receive enough total light. Raise the ISO only after you have opened the aperture and slowed the shutter as far as your subject and your ability to hold the camera steady will allow.