ISO and the Exposure Triangle
Much in photography revolves around light. The amount of light that hits your camera's sensor is determined by two settings: the aperture and the shutter speed. The aperture determines how large the opening of your lens is, while the shutter speed determines how long the sensor is exposed to light.
The third setting within the exposure triangle is your ISO. Unlike the aperture and shutter speed, the ISO does not change how much light hits the sensor. Instead, the ISO setting affects how the signal from the sensor is amplified and processed.
This may sound a bit technical, but the principle is actually quite simple. With a higher ISO value, you can take a brighter photo without having to change the shutter speed or aperture. For example, if you take a photo with a shutter speed of 1/250 second, an aperture of f/4, and ISO 100. When you then increase the ISO to 800, the photo becomes significantly brighter. The amount of light captured remains exactly the same, but the camera processes the signal differently.
What is ISO exactly?
The term ISO comes from the International Organization for Standardization. This organization establishes international standards, including standards for the light sensitivity of photographic film and the ISO settings of digital cameras.
In analog photography, the film used determines how light-sensitive the material is. In digital photography, this works differently. The sensitivity of the sensor does not simply change when you increase the ISO. Instead, the camera adjusts the gain and processing of the image signal accordingly.
Still, ISO is often colloquially described as the light sensitivity of the camera. This is an easy and convenient simplification, as long as you understand that a higher ISO does not mean your camera actually captures more light.
Most cameras work with ISO values such as 100, 200, 400, 800, 1600, 3200, and so on. Each doubling means an increase of one exposure stop. When you go from ISO 100 to 200, the image at the same shutter speed and aperture becomes about twice as bright. At ISO 400, it is four times as bright as at ISO 100.
What happens when you increase the ISO?
A higher ISO value is especially useful when you have to work in low light. Think of a concert, a dark street, or a sports event in a gym. In these situations, you usually also want to use a shorter shutter speed to freeze movements. But with a shorter shutter speed, relatively little light hits your sensor. As a result, your photo can become too dark.
Then you can do a few things. Initially, you can extend the shutter speed, but then you run the risk of movements becoming blurry. You can also open the aperture further, but not every lens can open wide enough, and you might want to maintain more depth of field. In that case, ISO comes in handy. By increasing the ISO value, you can maintain the desired shutter speed and aperture while still achieving usable brightness.
Imagine you are photographing a moving subject. You choose a shutter speed of 1/500 second to freeze the motion well and an aperture of f/2.8. At ISO 200, you see that your photo is way too dark. By increasing your ISO to, for example, 3200, you can make the photo brighter without having to adjust the other settings. This does not immediately mean that a higher ISO is always the best solution. It remains important to look at which settings you need for your subject.
High ISO and Image Noise
One of the most well-known disadvantages of a higher ISO value is image noise. These are small grainy structures or colored spots that can become particularly visible in the darker parts of your photo. However, it is not entirely accurate to say that a higher ISO automatically results in poor image quality. Noise is actually very much related to the amount of light that the sensor actually captures.
When there is little light available, the image signal contains relatively more noise. By increasing the ISO, not only is the usable signal amplified, but the existing noise can also become more clearly visible. Therefore, you often see more noise in photos taken in dark conditions than in photos taken during the day.
The amount of noise you ultimately see varies greatly by camera. Modern sensors and image processors can often handle higher ISO values quite well. The size of the sensor also has an impact, although the amount of light captured and the settings used also play an important role.
That’s why it’s wise to discover for yourself which ISO values yield acceptable results for your camera. For example, take a number of photos in low light and view them on a larger screen. This way, you can see how many details are preserved and when the noise becomes truly distracting. Moreover, a bit of image noise doesn’t have to be a problem at all. Especially in street photography, concert photography, and black-and-white images, a grainy appearance can actually fit well with the atmosphere.
Set ISO manually or use Auto ISO?
Many beginners receive the advice to keep the ISO as low as possible. This is not a bad starting point in itself, but it does not mean that you should always set your ISO manually. Modern cameras have Auto ISO. This allows your camera to automatically choose an ISO value based on the available amount of light and the other settings.
Especially in changing lighting conditions, this can be useful. Instead of constantly adjusting the ISO yourself, the camera can take care of this. This way, you can focus more on your composition and the right moment. Moreover, with most cameras, you can set how high the automatic ISO can go.
Some cameras also offer the option to set a minimum shutter speed when shooting in aperture priority mode. For example, you can choose a maximum ISO of 6400 and a minimum shutter speed of 1/250 second. The camera then tries to find a suitable exposure within these limits. In low light, it may still need a longer shutter speed when the maximum ISO is reached.
Manually setting your ISO is of course still useful. When working with controlled lighting or photographing landscapes from a tripod, a fixed ISO value is often more practical. This way, your settings and results are easier to predict.
Which ISO value do you choose in different situations?
The best ISO value to use depends on the amount of light, your subject, and the settings you need. For many cameras, ISO 100 is a good starting point when there is enough light available. However, there are plenty of situations where a higher value makes more sense.
For example, when photographing a landscape during the day, ISO 100 to 200 is usually fine. There is often enough light available. Also, for portraits taken outdoors, ISO 100 to 400 is usually a good starting point. However, if you are photographing on a cloudy day or in the shade, it may be necessary to raise your ISO slightly. Think of ISO 400 to 800. This way, you can still often use a shorter shutter speed without your photo becoming too dark.
Are you photographing indoors without a flash? Then you will quickly end up with an ISO value between 800 and 3200, depending on the amount of light and your lens. At concerts and handheld evening photography, even ISO 1600 to 6400 or higher may be needed to capture movements sharply.
With night photography from a tripod it is different. When photographing a stationary subject, you can often work with an ISO value between 100 and 400. You can then use a longer shutter speed to still capture enough light, without having to raise the ISO significantly.
All these values are not fixed rules. A fast lens, for example, can allow you to photograph with a lower ISO, while a moving subject may require a higher value.
ISO 100 doesn't always have to be the best choice. Some cameras have a different standard or base ISO at which they deliver the best image quality. The most important thing is to choose the ISO value that fits the photo you want to take, and not solely based on the amount of noise you are trying to avoid.
Discovering how ISO works
The best way to understand ISO well is to experiment with it yourself. Choose a subject that doesn't move and photograph it under constant lighting conditions, for example indoors with a lamp. Then set your camera to the manual mode and choose a fixed aperture and a fixed shutter speed. Then take a number of photos where you only change the ISO. Start with ISO 100 and gradually increase it to 200, 400, 800, and 1600. When you place these images side by side, you will see that the brightness increases progressively.
Do you want to investigate what ISO does to image quality? Then make a second series where you increase the ISO while simultaneously shortening the shutter speed to keep the brightness approximately the same. In this series, the sensor captures less light, allowing you to compare how the amount of noise changes.
Ultimately, it's all about finding the right balance with your ISO. Of course, you want to retain as much detail as possible and avoid unnecessary noise. But a sharp photo with a bit of noise is often much more useful than a blurry photo taken at ISO 100. So don't hesitate to raise your ISO when the situation calls for it.
- Also read: The exposure triangle
- Also read:How to manually set your exposure settings
- Also read: The different modes in which you can photograph
