Photography 101: Exposure, aperture, shutter speed, and ISO

Unlock your camera's potential! Here's everything you need to know about exposure settings

Every good photo begins with the right exposure. Even if you catch a great subject at the perfect moment with strong framing, everything is lost if you blow the exposure.

If you only shoot in automatic mode, you’re accustomed to the camera taking care of all the settings. But, as adept as modern cameras have become, they aren’t perfect, and they’re not good at making artistic decisions.

Elevating your picture-taking from good to great requires a general understanding of the three elements of exposure: aperture, shutter speed, and ISO. Understand the interplay between these three elements, and you will be able to anticipate great photographs, rather than wait for happy accidents.

Photography 101

When you have a basic understanding of aperture, shutter speed, and ISO – which are also the basics of photography, in general – you’re well on your way to mastering your camera, even if you never open the user manual.

What is aperture?

The aperture is simply the opening within the lens that limits the amount of light that can pass through it. By changing the aperture value, you increase or decrease the size of that opening, thereby allowing more or less light into the camera.

Aperture is measured in f-stops, such as f/16 and f/4, but here’s the thing: The smaller the f-stop number, the larger the opening, and vice versa. So, when you are adjusting the settings, think of the opposite: If you want less light to enter (small aperture), go for a larger f-stop. How large your lens’ aperture can open will depend on your lens. (Hint: A lens’ maximum aperture will be part of its model name, like a 50mm f/1.8 or a 24-120mm f/4.)

Beyond controlling the amount of light, aperture determines an image’s depth of field (DOF). Simply put, DOF is how much depth will be in focus within the image. An image with a large DOF will have sharp focus from foreground to background, while a small, or shallow, DOF sees the focus concentrated on one particular plane, with foreground and background elements blurred away.

When thinking about the f-stop, choose a smaller number (larger aperture) to achieve a shallower DOF, or a larger number (smaller aperture) to increase DOF.

Hasselblad XCD 21mm Samples
A small aperture (larger f-number) is used to keep both the foreground and background in focus. Download full resolution

When would you want to control aperture? Almost always. The most common examples are portraits and landscapes. Portraits often look more appealing when the subject is separated from the background, which a shallow DOF will achieve.

On the other hand, for landscapes we typically want everything to be in sharp focus, from the foliage in the foreground the distant mountains. If you’re not sure how much depth of field you need, the beauty of digital photography is the ability to “guess and check.” Simply take a photo, check it out on the camera’s LCD screen, and either increase or decrease the size of the aperture to get the desired DOF.

A large aperture (small f-number) can separate the subject from background and/or foreground. Daven Mathies/Digital Trends

What is shutter speed?

Just like the shutters on a window, a camera’s shutter opens to allow light in. The shutter lives just in front of the imaging sensor, and the shutter speed is the amount of time it stays open, like 1/60 second.

Aperture and shutter speed work together. Whereas the aperture determines the amount of light that’s coming through the lens, the shutter determines the length of time the sensor will be exposed to that light. When you set the shutter speed – usually measured in fractions of a second (e.g. 1/30, 1/1,000) – you are telling the camera how quickly or slowly to open and close the shutter. A shutter speed of 1/4,000 second is very fast and will let in very little light, while a shutter speed of 1/2 second will let in a lot of light.

In addition to its role in exposure, shutter speed controls how motion is captured by the camera. A fast shutter speed will freeze moving objects in their track, while a slow shutter speed will record the movement, allowing objects to blur. While a certain shutter speed is required to take a steady picture without a tripod, blur isn’t necessarily bad and sharpness isn’t necessarily good. There are many situations when the choice between the two is a creative one, rather than a technical one.

For example, imagine a race car: Some may want a blurry effect to illustrate its motion around the track, while others may want to freeze it to show a specific moment, such as when it crossed the finish line. In the former example, you’ll want to try a slower shutter speed, like 1/60, while the latter example would require a speed of 1/1,000 or more.

Even slower shutter speeds lead to what’s called long-exposure photography. This is how you can blur things like waterfalls or create star trails in the night sky.

On a tripod, a slow shutter speed will add motion blur to any moving elements in the frame, such as water and foliage, while stationary objects will remain sharp.  Daven Mathies/Digital Trends

There are a few things to keep in mind. If you are using a very slow shutter speed, make sure your camera is stabilized on a tripod or other steady surface to prevent camera shake. When hand holding your camera, the slowest shutter speed you can shoot without introducing shake depends on many factors, including the focal length of your lens and whether or not it (or your camera) has image stabilization. Generally speaking, shutter speeds between 1/60 and 1/125 second (or faster) are safe for handheld shots.

What is ISO?

ISO is the short name given to the International Organization for Standardization, but its meaning in photography is unique. Also known as “film speed,” it is a rating carried over from the film days, but means the same thing on digital cameras. ISO controls how the sensor responds to the light it receives from the shutter and aperture. A high ISO leads to a brighter image, whereas a low ISO will be darker.

Aperture, shutter speed, and ISO all affect exposure in similar ways, but whereas the first two have creative side effects (DOF, motion blur), ISO isn’t so lucky. Generally speaking, you’ll want to keep ISO as low as possible while still achieving the correct exposure, as increasing the ISO also increases noise. When you hear someone describe a photo as being “too noisy,” ISO is likely to blame. Noise is rarely a creative effect we’re after, and if we are, it’s usually best to add it in post.

Sensors also perform better at low ISO settings, recording better color depth and dynamic range. (In the film days, high ISO films were “grainier.” You can think of “grain” and “noise” as being effectively the same thing.)

In daylight or well lit scenes, the ISO can be set low, usually around 100 to 400. In such cases, you can typically rely on just your aperture and shutter speed to dial in a proper exposure. However, in dim lighting, there may be no other option than to increase ISO. Furthermore, if you want both a fast shutter speed and deep depth of field (small aperture), then raising the ISO might be necessary.

ISO is reflected by a number that, unlike shutter speed, does not reflect any sort of real-world measurement. Most cameras start at ISO 100 or 200, and can be set up to 12,800 or higher. The math is, fortunately, simple: Double the ISO, and you double the brightness, so ISO 800 is twice as bright as 400, which is twice as bright as 200, etc.

Many cameras advertise insanely high maximum ISOs, but don’t always believe the marketing hype. Just because a camera can shoot at ISO 102,400 doesn’t mean you should use it.

A dive bar makes for a challenging low-light scene. Even with a slow shutter and wide aperture, a high ISO of 12,800 was required, leading to visible noise. Daven Mathies/Digital Trends

Is there a way to shoot in dark settings without raising the ISO? Well, there’s always flash – but in general, on-camera flash produces unflattering results. Whether you prefer the look of flash or the look of noise is a choice you may have to make.

You can also place the camera on a tripod to allow the use of a slower shutter speed, thereby keeping ISO low. However, if you’re trying to shoot people other other subjects that aren’t perfectly still, this may not be an option. Finally, investing in a lens with a wider maximum aperture might be a good idea, as the kit lenses included with most interchangeable lens cameras have relatively small maximum apertures. But, as evidenced in the photo above, sometimes even an f/1.4 lens isn’t enough to keep the ISO down.

How do aperture, shutter speed, and ISO work together?

As you’ve probably concluded, aperture, shutter speed, and ISO work together to properly expose an image. A change to one will impact the other two. For example, increasing the shutter speed will need to be matched by an increase in ISO or aperture size to maintain the same exposure value. Likewise, decreasing the size of the aperture (selecting a larger f-number) will require a faster shutter speed or lower ISO.

The basic point here is that selecting an exposure is always an exercise in compromise. In many situations, you’ll be able to choose the exact settings you want, but in tricky lighting, finding a balance of settings that offers the DOF, sharpness, and noise levels you want may be a challenge.

If this is sounding complicated, don’t worry: In addition to full automatic and full manual exposure modes, there are “in-between” modes that give you more control while streamlining and simplifying the exposure process. Want to only worry about depth of field? Aperture Priority or Aperture Value mode (A or Av) lets you set the aperture and compensates with the shutter speed automatically. Shutter Priority or Time Value (S or Tv) is the reverse, letting you manually select a shutter speed while the camera controls the aperture. Auto ISO is generally turned on or off independently of exposure mode, and turning it on will remove another step from the equation, but may inadvertently lead to noisier pictures.

Don’t freak out if this all seems overwhelming. Know that there’s no magic answer to understanding exposure, and even professional photographers constantly play with the settings until they get an image they are happy with. The key is to experiment until you become familiar with the different effects.

For this post-sunset scene, a combination of settings allowed for the sharpness and depth of field we needed, while not introducing too much noise. (1/125 sec., f/5.6, ISO 800). Daven Mathies/Digital Trends

Key takeaways

  • When it comes to the aperture setting, think of the opposite: A small f-stop number equals a large aperture setting, while the large f-stop number equals a small aperture setting.
  • To blur the background of a portrait, choose a large aperture (small f-number). To keep the entire image in-focus, choose a small aperture setting (large f-stop).
  • To “freeze” a moving object, use a fast shutter speed. For a blurring effect, use a slower shutter speed.
  • A tripod is handy when shooting images at very low shutter speeds.
  • To reduce noise, try to always keep ISO as low as possible — but know you may have to raise it in low light settings.
  • Changing one setting affects the others. If you are using a slow shutter speed, you’ll likely need to use a smaller aperture to compensate.
  • If you don’t mind handing over some control to the camera, choose Aperture Priority mode to only control depth of field or Shutter Priority mode for motion capture.
  • There’s no wrong way to take a photo. Experiment with the settings until you get the shot you’re happy with.

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