Exposure time, aperture and light sensitivity are other terms for shutter speed, f-stop and ISO, respectively. Part of the art of photography lies in finding the right balance among these three settings. By adjusting these three elements of exposure, you can achieve very different results.
Originally, “exposure” had a very literal meaning: photographic film is sensitive to light, so making a photograph simply meant exposing the film to light for a short period. Put simply, an exposure is the image recorded when the camera’s shutter opens and exposes the film—or, today, the camera’s image sensor—to light.
Three primary variables therefore shape an exposure. Together they are known as the “exposure triangle,” because all three are equally important and each affects the exposure in a different way.
Shutter speed
Shutter speed refers to the length of time the camera sensor is exposed to light. It is measured in seconds or fractions of a second. A slow shutter speed (e.g. 1 second) lets in more light, which is ideal for low-light situations or for creating motion effects, such as blurring flowing water.
A fast shutter speed (e.g. 1/1000 second) freezes movement, perfect for capturing fast action such as sporting events. That very short time is enough for the image sensor to take and record the photo, provided the other two variables are adjusted correctly.
The amount of time the shutter remains open also affects how an image looks. For example, very fast subjects shot at very fast shutter speeds look frozen in time. Even water droplets can appear razor-sharp in the air, provided you choose a shutter speed fast enough to capture them that way.
On the other hand, if you use a slow shutter speed, you will capture motion blur. Sometimes this is undesirable, but motion blur can also be used creatively. For example, very slow shutter speeds can capture star trails or light trails from cars, which appear as long streaks in the final image. This can add a lot of creativity to taking a photo.
Long shutter speeds are only a problem if there is movement in the shot and/or if you have not mounted your camera on a tripod. Trying to hand-hold the camera during a long exposure can cause "camera shake," or visible blur caused not by the movement of the subjects, but by the movement of your hands. If the camera is mounted on a tripod, you can use long exposures on almost any stationary object, but anything that moves will still appear out of focus, even when the camera is stabilized.
Aperture
The aperture is the opening in the lens that can be enlarged or reduced to control how much light reaches the sensor. It is expressed as an f-number, such as f/2.8 or f/8. A low f-number means a larger opening and more light, while a high f-number means a smaller opening and less light.
Aperture differs from shutter speed in that it is related to the size of the opening through which light reaches the sensor, rather than the amount of time the sensor is exposed to that light. So with a larger aperture you can take photos in low light, and with a small aperture you can take photos in bright light, but there is much more to it.
With large apertures (low f-number) you can use fast shutter speeds, because with a large aperture you need less time to get the necessary light onto the image sensor. Small apertures (high f-numbers), on the other hand, require relatively slow shutter speeds for the opposite reason: generally less light passes through that opening, so the shutter has to stay open longer.
Aperture also has another crucial function: it controls depth of field. You have almost certainly heard the term, but if you are new to photography, you may have only a vague idea of what it means.
Simply put, depth of field is the amount of the photo that remains in focus. A larger aperture (smaller f-number) provides a shallower depth of field, blurring the background and putting the focus on the subject.
• Large aperture: A photo with a shallow depth of field may have an area of just a few centimeters with very sharp focus.
• Small aperture: A photo with a large depth of field can have sharp focus from foreground to background.
ISO
The third element in the exposure triangle is ISO. ISO is the sensitivity of the camera sensor to light. A lower ISO value (e.g. ISO 100) is less sensitive and ideal for bright conditions. A higher ISO (such as ISO 3200) is more sensitive and better suited for low-light situations, but may lead to more noise or grain in the photo.
ISO is actually an older term derived from the International Organization for Standardization. It dates back to the early days of film, when manufacturers did not share a standard for film sensitivity, making it difficult for photographers to know which films suited low light and which did not. In 1974, film-speed standards were unified across brands, so a roll of Kodak 100 was roughly equivalent in light sensitivity to a roll of Fujifilm 100.
The term is still used in digital photography to describe light sensitivity. Today, you no longer have to change a roll of film to change sensitivity; you simply adjust a setting in your camera’s menu. That means you can make one exposure in very low light, then step outside and use the next frame to photograph a sunny landscape.
What matters to you is how ISO affects an image. You already know that this makes your camera more sensitive to light, so one of the most valuable things you can do with it is take photos in low light without using a tripod. High ISOs can also help you achieve a fast shutter speed in semi-bright conditions, which is important when you're trying to capture very fast action in less than optimal shooting conditions.
But high ISOs also have disadvantages, especially with older camera models. When you set the ISO too high, you can get digital noise in the photos, which manifests itself as a kind of sandy or grainy texture.
Very high ISO settings can produce images with long bands of discoloration, noise and a grainy appearance. Less obvious are the effects on color and tone, which can become increasingly muddy or poorly defined as ISO rises.
But this is actually a bigger problem for older camera models; new models can handle high ISOs very well and you may not be able to easily tell the difference between a photo you took at ISO 400 and a photo you took at ISO 3200.
Practical tips for beginners
Start with Auto mode: If you're just starting out, experiment with your camera's automatic mode first. This helps you get a feel for how the camera determines exposure.
Practice with one setting at a time: Try experimenting with one setting first, such as just the shutter speed, while keeping the other on auto. This helps you understand how each setting affects the photo.
Use the ISO: Don't be afraid to play with the ISO. Modern cameras have excellent capabilities at higher ISO values, giving you more flexibility in different lighting conditions.
Pay attention to motion blur and noise: Slow shutter speeds can cause motion blur and high ISO values can cause noise. Be aware of these effects and how to minimize them.
Conclusion
You can think of an exposure as the result of the different ways your camera presents light to the image sensor. First there is the size of the aperture—the opening between the lens and the sensor. Next comes the length of time light is allowed through that opening, and finally the sensor’s overall sensitivity to the incoming light. Together, these three factors determine the exposure: the image you capture in pixels.
Also read: From Auto Mode to Manual: Photographing with the Sunny 16 Rule
