Exposure – the dependence on aperture, time and ISO value

Exposure – the light’s path to the perfect shot

Three factors must always work together to create an image in your camera – regardless of whether you are using a DSLR, 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 reproduced clearly if light hits the sensor in the right quantity and dosage. In this article, you will learn about the three most important settings that shape your image: Aperture, shutter speed and ISO sensitivity.

The exposure trinity: aperture, shutter speed and ISO

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

The light passes through an opening (lens) through an aperture (1). This aperture is designed in such a way that it can sometimes be larger and sometimes smaller; the photographer can determine this himself. The shutter (2) allows the light to fall on the sensor (3)for a certain period 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. This is very helpful on the one hand, but 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 aperture

The aperture is usually built into the lens. Older lenses still have an aperture ring that can be turned to adjust the aperture. Nowadays, the aperture is often set via the camera. But no matter where you turn it, whether on the camera or on the lens, or perhaps 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 further you close the aperture, i.e. the smaller the opening, the less light gets through.

The size of the aperture is described with a number. I will explain exactly how these are arrived at 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 f-number (e.g. f2.8) means that the aperture is very wide open. A large f-number (e.g. f22) means that the aperture is very small. The “f” stands for the English term “focal length” – which makes no sense at this point. As I said, this will be a separate article. For now, we’ll just note that this refers to the aperture.

Shutter speed: duration of exposure to light

Time is an important factor in photography. I’m not talking about the right time to take a photo, that’s also important, I’m talking about 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 is opened and automatically closed again after a predetermined time. If you imagine the sensor as a glass that collects the light, then the exposure time is the time it takes for light to drip into the glass. Assuming that the light always ‘drips’ at the same rate, a longer exposure time collects more light than a shorter exposure time.

Interaction of exposure and aperture

It gets really exciting when you combine aperture and time. The one determines the other. To take the example of the glass again, it is easy to imagine that a certain amount of light can pass quickly through a large aperture. If the aperture is small, the same amount of light needs significantly more time.

The f-numbers, as well as the shutter speed, each describe one f-stop.

The aperture series is designed so that the amount of light falling through the lens halves from aperture stop to aperture stop
… if the next aperture stop has a higher value (e.g. 11 → 16) or
… doubles if the next aperture stop has a smaller value (e.g. 11 → 8).

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

Of course, I can do exactly the same in the other direction. If I need a faster time and change it on the camera, then I also have to make sure that the aperture is opened wider.

ISO sensitivity: The light sensitivity of the sensor

There is a third factor that is decisive for the exposure, the sensitivity of the sensor.

In analog photography, the photographer had to decide which film to use. If the weather was cloudy, a 400 film was often put 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 your 36 photos with it.

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

A low ISO value means that the sensor requires more light. A high ISO number means that less light is needed for a photo, the sensitivity increases. Incidentally, the human eye does the same. When we come from bright daylight into a dark apartment, the eye needs a short time to get used to the new lighting conditions – a new ISO value is set, so to speak.

I’ll risk a spoiler alert here: the lower the ISO value, the higher the image quality. Depending on the camera used, the image becomes increasingly grainy at high ISO values and the dynamic range decreases. The aim is therefore usually to work with the lowest possible ISO values.

Exposure time – Aperture – ISO

Up to this point, we have learned that there are three parameters that have an influence on a photo. Each of these parameters can be changed individually, but always has an influence on the other two settings. Optimal exposure can only be achieved if all three factors are set correctly. The following graphic shows once again the interplay between aperture, time and ISO.

When you use which time or aperture when taking photos depends on what you want to achieve. Both the shutter speed and the aperture can be used to compose the image. I go into 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 operation (M)
2. Deactivate the automatic ISO – select your own value instead, 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 correct the time until the exposure is correct.
5. Set the ISO value to 1600 and take another picture
6. Correct the aperture value until the subject is correctly exposed again.
7. Do this exercise as often as you can in different lighting situations. Always make sure that the camera is set to manual mode and the automatic ISO is switched off.

The aim of the exercise is not to have to think about how to change the values, but to intuitively know which controls you can turn to correct the exposure.