The exposure triangle: aperture, shutter speed, ISO

Master the exposure triangle, master photography

The basic elements that influence the exposure of a photograph are light, aperture and ISO. The light determines the amount of brightness available, the aperture regulates the opening of the lens and thus the amount of incoming light, and the ISO sensitivity affects the light sensitivity of the image sensor. To find the perfect balance between light and shadow, a thorough understanding of these factors is essential. Photographers need to master the nuances of these parameters in every situation. This is the only way to take precise pictures. We show you what is important.

Aperture, shutter speed, ISO

Here we take a closer look at the effects of aperture, shutter speed and ISO on your shots. We look at the positive and negative factors of the individual values - and show how they interact with each other. Because only together, and not on their own, do these factors play out their full effect to capture unique images. But don't worry, aperture, shutter speed and ISO are not that complicated. And once you have a perfect understanding of how these values interact, you can master all photographic genres.

Function and effect of the aperture on your pictures

Setting the aperture of a lens is one of the basics of photography. Not only can the aperture be used to control the amount of incident light, it also has a direct influence on the image result. However, the effect of the aperture and the way in which it is set can appear somewhat confusing at first. The reason for this is usually the so-called aperture scale (f/ scale), the nomenclature of which is initially contradictory to the function for many. You can find out more about this in the following section. First, we will briefly explain the actual function of the aperture:

How the aperture works is quickly explained: in simple terms, it is simply a hole in the lens through which light falls onto the camera's sensor. This opening consists of several blades that overlap - also known as aperture blades. These can be closed or opened more, depending on the f-number. This controls how much light is allowed through to the sensor. Aperture f/1 stands for an almost complete opening of the aperture with maximum light transmission, while aperture f/22, for example, only lets very little light through to the sensor due to its small aperture.

Consequently, a large aperture such as f/2 allows much more light to pass through than a small aperture such as f/16. Finally, it becomes somewhat confusing when it comes to designating the different states of the aperture: Why do the values jump from f/4 to f/5.6 to f/8? And why does a small value such as f/2 stand for a larger aperture than f/22? The answers can be found in the following paragraph.

The math behind the aperture value

The reason for the aperture designations such as f/1.8, f/4, f/11 or f/22 is simple: the f-number f/ is not a physical unit that could indicate the diameter of the aperture, for example. Rather, it is a dimensionless ratio that is calculated from the diameter of the aperture and the focal length. For example, f/4 means that the focal length (f) must be quartered in order to obtain the aperture dimension. At f/16, the focal length is divided by 16 to obtain the aperture.

At first glance, the order of the aperture values in which the f/ values are set on a camera also appears arbitrary. However, it also follows a rule: the amount of light falling through the aperture onto the sensor is determined by the aperture area. Accordingly, the aperture values must also be considered "square". f/2 squared equals 4, f/4 squared equals 16, etc.

In photography, the term f-stop has become established, which is related as follows: One f-stop more is associated with a doubling of the incidence of light. However, if the edge length is doubled, the area is quadrupled. Multiplying the edge length by "root 2" takes this fact into account; in this case, the area merely doubles. The f-stops result accordingly: f/2 multiplied by the square root of 2 gives f/2.8, and f/5.6 multiplied by the square root of 2 gives f/8. A simple way of remembering that higher values mean a smaller aperture is to imagine the f/ values as fractions; so f/8 would equal 1/8 or f/16 would equal 1/16.

What aperture has to do with depth of field

Caption: For portraits, a small aperture such as f/2 is often used to blur the background into an artistic blur.

Of course, the aperture is not the only factor that determines the exposure of a shot. The shutter speed and ISO sensitivity also have an influence on how much light is required to create an image. Automatic and semi-automatic modes on the camera make it possible to intervene selectively in the choice of values.

Although the lens and the camera can only ever focus on a very specific point, the choice of aperture has an influence on the sharpness of the image - more precisely, on the area in front of and behind the focused point that is still in focus. This area is known as the depth of field. A small aperture creates more depth of field than a large aperture. A large aperture can therefore be used to crop a subject more, as the background becomes blurred more quickly. At the same time, the exposure time can be shortened as more light hits the sensor through the aperture.

This has advantages for portrait and sports photography. However, focusing errors become more visible in the image. A smaller aperture reverses the effect accordingly: A larger image area appears sharp and focusing errors can be concealed more easily. A large aperture such as f/16 is therefore always used when a particularly large amount of detail is to be captured in the image, for example in macro or landscape photography. However, the exposure time usually has to be extended accordingly to get the right exposure. In any case, steady hands, an image stabilizer or even a tripod are usually required for long exposure times or more closed apertures.

Beware of diffraction

BU: The larger the aperture value, the greater the depth of field. But be careful: too high a value can again lead to a slight overall blur.

The smallest aperture, such as f/2.8, of a lens generally provides the greatest depth of field for your shots. However, this can also result in a slight blurring of the entire image. This optical phenomenon is known as diffraction. It occurs because the incident light is refracted at the hard edges of the aperture blades and thus deflected. In principle, diffraction occurs at any aperture; however, the smaller the aperture (e.g. f/22), the more pronounced the effect.

Sharpening the image in post-production on the computer can bring a certain improvement here, but qualitatively perfect results can be achieved if the aperture is opened a little wider - for example to f/16 instead of f/22. In addition, the shutter speed is usually slower with a very closed aperture. As the shutter speed has to be increased accordingly in this case, motion blur can also occur in the image. Alternatively, you can also open the aperture a little wider, use optical image stabilization or use a stable tripod to always get the sharpest shots in the respective situation.

Beautiful points of light: How the aperture blades influence the bokeh

The number of aperture blades of a lens has a direct influence on the quality of the blurred areas in the image, the so-called bokeh. As a rule of thumb, the more aperture blades, the better, as they create a rounder light disk. The number of aperture blades can be found in the technical data of your lens: As a rule, between five and nine blades are installed. In addition, the larger the aperture is selected, the rounder the light circles in the background will appear. When stopping down to a smaller aperture, the actual shape of the aperture becomes clearer in the image. This can result in unsightly edges and corners. Circles of light are always easy to recognize when small light sources are placed in the blurred background. At the smallest aperture, you will not see discs of light, but stars with small peaks corresponding to the number of blades.

Note: If you photograph with a large aperture, points of light are always displayed as round shapes. If the lens is stopped down, the number and shape of the aperture blades become visible.

Setting the shutter speed masterfully

BU: Fast shutter speeds help you to freeze moving subjects in action.

The shutter speed, also known as the exposure time, is the second important factor in the exposure triangle. It determines how long light falls on the sensor - which also determines whether a subject in the picture will be in sharp focus or blurred.

A photo is only really sharp if the contours and details of a subject are clearly visible. To achieve this, two conditions must be met when taking photos: Firstly, the light rays must be bundled exactly on the sensor plane. It must therefore be correctly focused. Secondly, the subject should not move during the shot if possible. If this is the case, the photographer can counteract this to a certain extent with a very short exposure time.

The photographer must therefore always weigh things up: How fast does the shutter speed need to be in order to capture a moving scene as a pin-sharp "still image" on the memory card? Relatively slow shutter speeds of 1/125 of a second and slower can be used by landscape and architectural photographers as well as photographers of people sitting still for a photo. When photographing sporting events, animals or even children playing, on the other hand, you are well advised to reduce the shutter speed to at least 1/500 of a second, and even faster for fast, often impulsive movements, as otherwise the main subject will appear blurred.

Consider shooting distance and focal length

However, choosing the right exposure time is not that easy. The decisive factor is not the speed of the subject, but how far it moves in the respective image section during the exposure. Take soccer, for example: If you have a relatively short focal length and can see almost the entire playing field, you can choose a slightly longer exposure time of approx. 1/250 sec, as the individual players are only relatively small in the image section and only change their position by fractions of a millimeter. On the other hand, if you want to use your telephoto lens to capture a goalkeeper parade in the right light, an exposure time of more than 1/1000 of a second is a good idea to freeze the acrobatic leaps without "blurring".

Conversely, this means that if you're shooting with a wide-angle lens, you don't need to worry so much about exposure times. If, on the other hand, you are using a telephoto lens, you should pay attention to short exposure times. This applies all the more if you are photographing without a tripod, because even if the subject is not moving - your hand may be moving, even if only minimally. This slight wobbling or trembling is particularly noticeable at high magnification.

Practical rule of thumb: the freehand limit

A tried and tested photographer's rule will help you to estimate your "freehand limit": The shutter speed should not be slower than the reciprocal of the effective focal length. For a 200 mm lens on a DSLR with an APS-C sensor, the following applies: crop factor 1.5, multiplied by 200 mm results in an effective focal length of 300 mm. The shutter speed should therefore not be slower than 1/300 s. If you have a steady hand or equipment with image stabilization, you can increase the shutter speed a little, but everyone else will be fine if they stick to this rule of thumb.

Sufficient light on the sensor

The exposure time not only determines the sharpness of an image, but also the amount of light that hits the sensor. The shorter the exposure time, the darker the image will be. Unless the lower amount of light is compensated for in another way, e.g. by opening the aperture or using a higher ISO sensitivity. Achieving a balanced exposure in manual shooting mode ("M") requires some experience. With almost all system cameras, however, it is also easier. This is with the semi-automatic shooting modes "Tv" ("Time Value") or "S" ("Shutter Priority"), also known as "Time Priority". Here, the photographer simply sets the exposure time. The camera automatically determines the correct aperture in order to obtain a sufficiently exposed image. Depending on the camera model, this mode is therefore sometimes also referred to as "aperture priority".

This shooting mode is highly recommended for anyone gaining their first experience with photography. It allows you to concentrate fully on the interplay of exposure time and sharpness. The camera takes care of the rest.

Professional photographers prefer to compensate for the lack of light themselves in manual shooting mode. They are familiar with the "exposure triangle" with the three parameters that influence image brightness: Aperture, shutter speed and ISO. If you halve one of the three parameters, for example reducing the shutter speed from 1/125 second to 1/250 second, you have to double another parameter to compensate, for example opening the aperture from f/5.6 to f/4.0 or increasing the light sensitivity from ISO 400 to ISO 800. The amount of light then remains the same and the shorter exposure time does not affect the image as underexposure.

Which exposure time to use and when: Tips and tricks

Long exposure with creative blurring

Caption: Landscape photographers rely on long exposure times to capture flowing water in a silky blur while the rigid landscape remains sharp.

Most photographers want razor-sharp images. Others enjoy blurring the image to create dynamic accents or make motion sequences visible. One example of this is light painting shots at night, where the headlights of passing cars can be seen as glowing stripes on the subject thanks to a longer exposure time.

Motion blur is also very suitable for emphasizing the speed of an object. For example, when the spokes of a bicycle appear blurred. However, make sure that other important elements, such as the frame of the bike and the rider, are in focus, otherwise the image will look unprofessional.

You can also use the long exposure to capture sights "without tourists": Visitors usually only stay in one place for a few seconds. With an exposure of several minutes, they therefore disappear - as if by magic - from the scene. To take such a shot in daylight, the amount of light must be drastically reduced. To do this, a gray filter (neutral density filter) is screwed in front of the lens.

With most camera models, the maximum shutter speed that can be set is 30 seconds, sometimes a shutter speed of 60 seconds is also possible. If you need an even longer exposure time, you can often activate the "Bulb" mode. The shutter remains open for as long as the shutter release button is pressed.

Capturing fast movements with a fast shutter speed

U: To freeze subjects in motion, you need particularly short exposure times.

Short exposure times are ideal for making extremely fast processes visible that are no longer perceptible to the human eye. Examples of this are the flapping of a hummingbird's wings or the bursting of a "water bomb". This requires extremely short exposure times of around 1/8,000 of a second. However, so little light falls on the sensor in this short period of time that artificial light has to be used to help. Flash units with "High Speed Synchronization" (HSS) are used for this purpose.

Motorsport photographers also rely on particularly fast shutter speeds. However, it is difficult to flash here, as the distance to the racing cars on the ring would be too far - and the subjects too large. They counteract this with the third, important value in the exposure triangle: the ISO. You increase this to slightly raise the basic exposure of the image. However, there are also a few pitfalls to be aware of here.

Image noise under control: How light sensitivity (ISO) affects images

Caption: Astrophotography and night photography are the classic situations where a higher ISO value is necessary due to the low ambient light.

ISO dates back to the days of analog photography and stood for the light sensitivity of the film. The light sensitivity of negative films was an important factor in analog photography. You thought carefully beforehand about which subjects you wanted to photograph in the next 24 or 36 shots and tried to estimate the lighting situation. You then decided on a fine-grained negative film with ISO 100 or a more light-sensitive one with ISO 800 and had to live with this decision until the film cartridge was full.

Digital photography makes it easier: the ISO setting is no longer a fixed value, but can be adapted to the subject from image to image. Today's cameras offer low-ISO settings as well as high-ISO settings that are so high that in many cases they make the flash superfluous. This function gives the photographer much more freedom in the choice of aperture and shutter speed. But there is a catch: the higher the ISO selected, the more noise is introduced into the image.

While special coating processes increase the light sensitivity of negative films, the signal is simply amplified in camera sensors. With a higher ISO sensitivity, the same amount of incident photons is therefore interpreted more brightly. This is important in all situations in which the available light is the limiting factor and the photographer does not want to open the aperture wider or extend the exposure time for creative reasons. In practice, this means that if you double the ISO value, you can reduce the aperture or shutter speed by a factor. So if you increase the ISO sensitivity from 200 to 400, you can either reduce the shutter speed from 1/125 s to 1/250 s or close the aperture from f/5.6 to f/8 without the image becoming too dark.

Beware of high ISO

Digital technology offers a very wide range of ISO settings. In contrast to negative films, where ISO 3,200 is the upper limit, image sensors play in a different league: ISO 102,400 is no longer a record value, this setting is offered by many current models.

Despite all these improvements, one basic rule of photography still applies: Only set the ISO sensitivity as high as necessary. Although increasing the signal leads to a higher image brightness, it also leads to a higher background noise of the sensor, known as "image noise" in photography.

This noise is made up of two components: With luminance noise, some pixels appear too bright or too dark; with color or chrominance noise, they show a completely different, i.e. "wrong" color. When taking pictures with high ISO values, a colorful confetti rain can fall on dark image areas. The extent to which the brightness and color noise has an effect and whether it forms large patterns varies depending on the camera model.

A similar effect occurs with extreme long exposures. This is the case in astrophotography, for example. Here, the background noise adds up due to the slow shutter speed. With many cameras, this noise can be removed internally using the so-called "dark frame subtraction": after the actual exposure, the camera takes a second shot of the same duration, but with the shutter closed. This "dark frame" shows the specific pattern of the image noise. It can be used as a template for filtering. This produces astonishingly clear images of the starry sky.

The high light sensitivity not only favors image noise, but also has another, less obvious disadvantage: the dynamic range of the images decreases rapidly.

The right ISO: practical application examples

ISO 6,400 or ISO 200? There is no general answer to this question; it depends on the subject and lighting conditions. We have put together a few typical examples here to make it easier for you to assess the ISO value.

Motion blur

With an exposure time of several seconds, a waterfall turns into a veil. This can be achieved with a low ISO sensitivity.

Fine-Art

To reproduce the smallest details in the best quality, you should preferably work with a low sensitivity of ISO 200 or less. The lowest ISO value can vary depending on the camera and sensor. Tip: Switch off "Auto-ISO", as the camera quickly resorts to high ISO values without your consent, especially in darker surroundings such as in this still life.

Portrait photography & fashion photography

Portrait photographers work with an open aperture in order to crop people against a blurred background. High ISO sensitivities are usually not necessary for this. Tip: Use the lowest ISO sensitivity whenever possible to guarantee the best possible image quality.

Sports & action shots

Fast movements can only be frozen with very short exposure times such as 1/2000 of a second. This usually requires a higher ISO setting so that enough light falls on the sensor.

Concerts, events, indoor events

If the lighting mood on location is rather atmospheric and somewhat darker, the term "difficult lighting conditions" is often used. Here you usually have no choice but to increase the ISO, as neither longer exposure times nor a larger aperture are possible or would help. At concerts, there is no point in switching on the flash as the artificial light would destroy the atmosphere. It is better to boldly increase the ISO sensitivity.

Telephoto focal lengths: Animal and wildlife photography

When working with long focal lengths, such as in wildlife photography, short exposure times are often necessary to avoid blurring. To prevent the image from becoming too dark, higher ISO values often have to be used here too.

ISO automatic: quality assurance through limitation

If you don't want to keep an eye on the ISO yourself, you can delegate this task to your camera using the automatic ISO function. However, to prevent it from resorting to astronomical ISO values, it is advisable to limit the light sensitivity of the automatic function to a maximum value.

The "Auto ISO range" setting can be found in the camera menu. Here you can often also specify an exposure time that should not be undercut - important for hand-held shots.

Background knowledge: What is ISO actually?

The abbreviation "ISO" stands for "International Organization for Standardization". Since 1974, this standard has combined the previously valid ASA and DIN numerical values and describes the light sensitivity of negative and slide films. In digital photography, this designation for the light sensitivity of the sensor has been adopted, but unfortunately the standardization has been lost. Each manufacturer cooks its own soup, the ISO values of the various camera models are very different and therefore cannot necessarily be compared. What has remained the same, however, is that the higher the ISO, the more light-sensitive the sensor.