Exposure – the dependence on aperture, time and ISO value

Exposure – The path of light to the perfect shot

Three factors must always work together to create an image in your camera – regardless of whether you are using a digital single-lens reflex (DSLR) camera, a compact camera or a smartphone. Light is the central element that the camera ‘sees’. As with the human eye, the image in the camera is only clearly reproduced when the right amount of light hits the sensor. In this article, you will learn about the three most important settings that shape your image: aperture, shutter speed and ISO sensitivity.

The exposure triangle: aperture, shutter speed and ISO

The quality and mood of your image depend on these three factors. Each of these settings affects how much light falls on the sensor and how your image is shaped as a result.

The light passes through an opening (lens) via an aperture (1). This aperture is designed so that it can be larger or smaller, which the photographer can determine themselves. The shutter (2) allows light to fall on the sensor (3) for a certain amount of time. The sensor, in turn, can be adjusted so that it ‘sees’ better or worse.

All three variables are adjustable, depending on what the photographer intends to do. On the one hand, this is very helpful, but on the other hand, it can be a little confusing, especially at the beginning. So let’s take a closer look at the individual elements.

Aperture: The size of the light opening

The aperture is usually permanently built into the lens. Older lenses still have an aperture ring that you can turn to adjust the aperture. Nowadays, the aperture is often set via the camera. But no matter where it is turned, whether on the camera or on the lens, or possibly not at all because the camera does it automatically, an aperture is always involved. The aperture is a circular opening which, the larger it is, the more light it lets through. The more you close the aperture, i.e. the smaller the opening, the less light comes through.

The size of the aperture is described by a number. I will explain exactly how these numbers are calculated in another article. At first, the numbers are a little confusing because they seem to be chosen arbitrarily, but this is not the case. For now, it is enough to remember that a small aperture number (e.g. f2.8) means that the aperture is very wide open. A large aperture number (e.g. f22) means that the aperture is very small. The “f” stands for the English term “focal length” – which doesn’t make sense at this point. As I said, that will be covered in a separate article. For now, let’s just remember that this refers to the aperture.

Exposure time: duration of light exposure

Time is an important factor in photography. I don’t mean the right moment to take a photo, which is also important, but rather how much time I give the light to shine on my sensor.

The exposure time is controlled in the camera by the shutter. When the camera is triggered, the shutter opens and closes automatically after a predetermined time. If you imagine the sensor as a glass that collects light, then the exposure time is the duration during which light can drip into the glass. Assuming that the light always ‘drips’ at the same speed, you collect more light with a longer exposure time than with a short exposure time.

Interplay of exposure and aperture

It gets really exciting when you combine aperture and shutter speed. One determines the other. To take the example of the glass again, it’s easy to imagine that a certain amount of light can quickly pass through a large aperture. If the aperture is small, the same amount of light takes significantly longer.

The aperture numbers, like the shutter speed, describe an aperture level.

The aperture range is designed so that the amount of light passing through the lens is halved from one aperture setting to the next
if the next aperture setting has a higher value (e.g. 11 → 16) or
doubled if the next aperture setting has a lower value (e.g. 11 → 8).

Let’s assume that the exposure is perfect when aperture 4 is selected at a shutter speed of 1/125 second.
If the aperture is closed further to f5.6 but the shutter speed remains the same, this means that the image will be too dark. The aperture has been reduced, but the shutter speed has remained the same, so there is not enough light coming in.
However, if I now extend the shutter speed from 1/125s to 1/60s, everything is fine again and the ratio of shutter speed and aperture is restored.

Of course, I can also do the same thing in the opposite direction. If I need a faster shutter speed and change it on the camera, then I also have to make sure that the aperture is opened further.

ISO sensitivity: The light sensitivity of the sensor

There is a third factor that is crucial for exposure: the sensitivity of the sensor.

In analogue photography, the photographer had to consider which film to use. If the weather was cloudy, a 400 film was often used in the camera; if the weather was sunny, a 100 film was sufficient. Once you had decided on a film speed, you had to shoot all 36 photos with it.

Thanks to digital cameras, I can now decide what sensitivity my sensor should have for each photo.

A low ISO value means that the sensor needs more light. A high ISO number means that less light is needed for a photo, increasing sensitivity. The human eye does the same thing, by the way. When we come in from bright daylight into a dark room, our eyes need a moment to adjust to the new lighting conditions – it’s like setting a new ISO value.

A low ISO value means that the sensor needs more light. A high ISO number means that less light is needed for a photo, increasing sensitivity. The human eye does the same thing, by the way. When we come in from bright daylight into a dark room, our eyes need a moment to adjust to the new lighting conditions – it’s like setting a new ISO value.

Exposure time – Aperture – ISO

So far, we have learned that there are three parameters that influence a photograph. Each of these parameters can be changed individually, but always influences the other two settings. Only when all three factors are set correctly can optimal exposure be achieved. The following graphic shows the interaction between aperture, shutter speed and ISO once again.

When you take a photograph, the shutter speed and aperture you use depend on what you want to achieve. Both the shutter speed and aperture can be used to compose the image. I will discuss this topic in a separate article.

Exercise:

Grab your camera, preferably with a tripod, and choose any subject you like.

1. Set the camera to manual mode (M)
2. Deactivate the ISO automatic setting – instead, select your own value, e.g. ISO 200.
3. Set the aperture to f/8 and take a photo.
4. Is the image too bright or too dark? Then adjust the time until the exposure is correct.
5. Set the ISO value to 1600 and take another picture.
6. Adjust the aperture value until the subject is correctly exposed again.
7. Adjust the aperture value until the subject is correctly exposed again.

The aim of the exercise is to no longer have to think about how to change the settings, but to intuitively know which controls to adjust in order to correct the exposure.