Understanding How ISO Affects Images (2024)

Camera ISO is often taught as a part of the Exposure Triangle, along with Aperture and Shutter Speed. It basically determines how much brightness you want to add to your exposure. When you get more technical, photographic experts will debate the meaning of ISO in relation to exposure. I have read some photographers say that ISO has nothing to do with the exposure. Most articles and publications (print and web) define ISO as the camera’s sensor sensitivity to light and is a part of the exposure. Nikon also defines ISO as one of the elements that controls the exposure of an image and the sensor’s gain or sensitivity to light. Other photographers would say the same thing, but is there a right or wrong technical definition of ISO?

A Quick Introduction To ISO

ISO (International Organization for Standardization) is a technical standards committee that is based in the EU that oversees the implementation of standards in technology. Before digital photography, the speed of the film was a measure of the film’s sensitivity to light. The higher the value, the more sensitive it was. The name ISO was adopted to show how sensitive the chemicals used in the film is to light ( I will not discuss the film process any further, but this link is a good reference about film). The ISO naming convention began to be used to define the film speed standards ASA and DIN. It was originally specified by an arithmetic or ASA number as well as a logarithmic or DIN number (e.g. 100/21).

The ASA number is a linear scale. The higher the number, the more sensitive the film is to light. The DIN number is on a logarithmic scale, so it uses a base 10 numbering system. DIN was originally used for black-and-white negative film speed ratings, but was added along with the ASA number by vendors. Later the DIN number would be omitted and film speed would be rated by ASA, which seems much simpler to understand. These two standards would then become combined into the ISO standards in 1974. This would then carry over from film to digital photography.

Using ISO In Digital Photography

The ISO rating would become a part of digital photography as a feature in digital cameras. The ISO rating for digital also uses a linear number system, but it has no direct relation to film. While the ISO refers to the sensitivity of light you need for the image, it is not a 1:1 relation to its film counterpart. In DSLR and mirrorless cameras, a higher ISO (e.g. ISO 800) is brighter compared to a lower ISO (e.g. ISO 100). A higher ISO is best for low light photography.

ISO can be set manually or allow the camera to automatically determine it. Many cameras have a default or base ISO value that starts at 100 (other camera makers have lower starting values). When you have enough light, like in sunny locations or when there is plenty of sunlight, the rule is to keep the ISO low. You do not need to change the ISO number since there is sufficient light hitting the sensor already. The rule for increasing ISO is in low light situations (e.g. night). Many event photographers who shoot in low light, will bump up their ISO when using flash photography. Higher ISO is more ideal when not shooting with a flash in order to capture more ambient light in an image. Although the purpose of ISO is more light sensitivity, artistic photographers will bump up their ISO to create certain effects which I will discuss later.

There is a drawback to high ISO, and that is more visual distortion or noise. When you increase the ISO in the image, it also amplifies the amount of noise. Noise is defined as graininess in an image due to the lack of light. It can best be described as a region in an image with inconsistent color and brightness, lacking details and contrast. When more light is hitting the sensor of a digital camera at higher ISO, if the scene is not well illuminated there will still be noise in the shadows and darker areas of an image. The result is poor quality due to the noise in the image. The higher your ISO, the more grains in the image. In film the noise in digital images would be synonymous with grain. The ISO does not create or add more light to the image, it is brightening it up through amplification which can also reveal the noise in an image.

Signal-to-Noise Ratio (SNR)

The problem of noise can be best explained by the SNR (Signal-to-Noise Ratio). This relates the signal strength to the noise that occurs in the sensor of the camera. The signal strength is the amount of power P. The signal in this case is the light that is hitting the sensor. Ideally the signal input strength must be stronger than the noise that is created by the sensor to have an acceptable output. This relationship shows the signal power to the noise power that is measured in decibels (dB). This is how to distinguish what is a wanted signal (useful information) in an image and unwanted signal (what is not significant).

When you have a higher SNR you are able to distinguish clearly the image from the noise. This is a good measure of clarity and ideal for high resolution images used for print and commercial work. However, if the SNR is low, the quality of the image suffers because if the signal and noise are equal or the noise has a higher value than the image is indistinguishable from the noise. Therefore, lower SNR values are not acceptable in most cases.

The thing to remember here is that the more light you have available, the stronger the signal, the less the noise. The less light you have, the weaker the signal, the stronger the noise. In order to get better image quality there has to be more light available or a stronger signal to illuminate the image. There are other sources of noise though, not just the absence of light. The sensor itself contributes to a natural noise which is a physical phenomenon of electronic circuits.

Fortunately for us, we do not need to calculate the SNR every time we are creating an image. It is good to know that if you see a camera sensor marketed with a high SNR capability, it means higher quality images. When it comes to real world use, it might be another story when it comes to performance, so don’t always trust the marketing at face value. Wait to hear from reviews and experts who have tested the camera.

How ISO Works

I am going to try to explain my own version of the technicalities behind how ISO works. Sensors are built with photosites, which capture the light. In MP (Megapixel) cameras, there are millions of these photosites which are a part of the sensor that captures the image. The light collected by the photosites provide a representation of the image illumination. When we talk about the sensor’s sensitivity it has to do with the amount of light that is hitting the sensor and whether the signal needs more or less amplification.

My understanding of ISO is based on what I was taught (from engineering classes and photography workshops) to what I learned (from experience as a photographer). There seems to be plenty of debate about what ISO really is, including the perspective of film and digital photographers. We can all agree that it has to do with “sensitivity” to light, whether it is film or sensor. I put sensitivity in quotes because that is the general understanding. The debate is to whether it has anything to do with the exposure or not. That is what makes this topic contentious.

When we set ISO values in our camera, we are either increasing its sensitivity to light by increasing the value or decreasing its sensitivity by lowering the value. That is the classic definition by most photographers. In most cases, we increase the ISO because we are shooting in low light conditions like during the night or in an indoor event where the lighting is not as bright. The important point to remember is how much light you need to capture the exposure and create the image.

In manual mode, what the camera does when we increase ISO is increase the sensitivity to light without slowing down your shutter speed or opening up your aperture (larger aperture, smaller f/stop value). Let us say you set your aperture and shutter speed manually to f/2.8 and 1/30, then those values will not change. Instead the sensor increases the amplification of the light signals or photons that hit the photosites. There is a limit though to how much of the signal can be amplified before it becomes too grainy, so that will also depend on the the aperture and shutter settings.

Let us try this experiment to show another way ISO works. Put the camera shooting mode to Program mode or P (check your camera documentation for the Program Mode). In P mode, you set the ISO and the camera determines the aperture and shutter speed. In this case the camera sets the exposure based on the set ISO value, which the photographer provides. I would not say that there is no consistency in exposure results since it will depend on the lighting conditions. For example when you set the ISO to 200, it will not be a consistent value set for the aperture and shutter speed for all cameras. It will depend on the lighting conditions of the photographer’s location. Even if you set your ISO to 600, but let us say the location actually has enough light, the camera will determine an aperture and shutter speed combination based on this.

ISO And Exposure

There is still the question, does ISO affect the creation of the exposure or not? Some photographers I have met say that ISO does affect exposure because it is part of the Exposure Triangle. Other photographers say that ISO has no effect on exposure, because exposure is a product of the aperture and shutter speed. I will attempt to answer this based on what I have learned.

ISO does not create the exposure. Although ISO is part of the image creation process or the Exposure Triangle, the actual exposure is determined by the light captured by the lens based on the aperture and shutter speed settings. What ISO does is control how dark or light the image will be based on the exposure settings. You can set your Exposure Compensation values, which are represented by stops in terms of EV units.

Increasing the EV units by 1 stop (indicated with a positive sign on the camera’s EV knob) , doubles the exposure. This means more brightness in the image so it will signal the camera to take a brighter exposure. Ideally this is how you would set your camera if you want lighter shadows and more highlights and vice versa (turn the knob toward the negative side of the EV). With a higher ISO setting, the light sensitivity is increased with more amplification at the photosites on the sensor. The exposure will always be the same because the amount of light captured by the camera is not changed by the ISO.

The ISO setting tells the sensor to amplify the signals. This is performed before the ADC (Analog-to-Digital Conversion) on the image. The analog capture of the image is converted to digital and processed by a DSP (Digital Signal Processor) in order for storage to the SD card in a digital camera. If this is a RAW image format capture, it will preserve most of the details in the image. The photographer can process the RAW image, including its dynamic range, color gamut and luminance based on the information recorded by the photosites. However, the full brightness on the image can be recorded as well, based on the ISO setting and exposure compensation value. Shooting in RAW gives a full dimension of the information that is contained and that can be retrieved using an image editing software.

Not everyone will have the same results. This is why this topic is so contentious. It also depends on the camera model, sensor size (e.g. cropped, full frame, medium format, etc.), sensor features, lens used, focal length, aperture and shutter speed settings just to name some of the factors. The best way to figure this out is to shoot, and learn what settings work best for the type of image you want to capture.

The Photoelectric Effect

What is sensor sensitivity? This is also another topic up for debate. The reason I stated “sensitivity” earlier is because the meaning can be misconstrued. The amount of light that hits the sensor is proportional to the electric charge that is captured. This is a part of what is called the Photoelectric Effect, which is the emission of electrons when radiation like light hits a surface i.e. sensor. This translates to an electrical charge or voltage that are recorded by the photosites which are then processed by an image processing chip. This gathers all the information recorded by the photosites, which represent the image to be stored and saved in digital media (e.g. SD card).

The “sensitivity” does not mean that the sensor changes in response to light. It doesn’t, it just records the values it is sensing from the amount of light it is receiving. One unit of light equals one unit of photoelectric charge. ISO values do not create more light for the sensor at all. The amount of light coming into the sensor is controlled by the aperture’s opening. The sensor’s features and capabilities were determined during the manufacturing process, not when creating the exposure. That is what sensitivity means in the context of ISO. The sensor always maintains the same level or degree of sensitivity but the level of amplification in the signal changes depending on the ISO value’s setting.

ISO And Image Quality

There is no one way or one size fits all setting for ISO. The best that a photographer can do is to experiment with the settings to come up with a decent exposure based on the aperture, shutter speed and ISO. These are just some tips for getting the best image quality when working with the ISO.

Consider your aperture settings. The best aperture setting must provide the desired DoF (Depth-of-Field). If you are going for bokeh, make sure you have a shallow DoF which requires a small f/stop which uses a larger/wider aperture. The thing to remember here is not to set your ISO too high since you already have a large aperture in which the opening of the lens is opened wider to allow more light. If your ISO is too high, the image will be overexposed. The key here is to balance the ISO with the aperture.

Use the base ISO value and use the shutter speed that provides the best exposure. This is more ideal when shooting with natural light, in well lit places or where there is plenty of sunshine. If you need more light, like when shooting in the shade, use light modifiers like reflectors to bounce more light to your subject. You may also want to use a tripod for slower shutter speeds that require more ambient light from the surrounding. Shooting handheld may cause blur, so using a stabilizing tool like a tripod will help capture a sharper and clearer image.

Generally, faster shutter speeds (e.g. 1/1000) are ideal for freezing motion but require plenty of light. You need fast shutter speed when shooting sports and fast motion events. In this case the base ISO is fine, especially if the lighting is good. When you need more light you switch to a lower shutter speed, allowing more time for light to hit the sensor so it has a wider aperture. In low light conditions, increasing ISO also helps to brighten the image. The point here is that fast shutter does not give more time for light to hit the sensor, which is why adequate lighting is necessary. When more light is needed, the shutter needs to be slower with a wider aperture and does not require a high ISO value.

When the exposure shows blurry images at the base ISO value, slowly try increasing the ISO matched with a faster shutter speed (e.g. ISO 200 at 1/400). Compose and capture the image, and check the LCD display to see if the image appears sharper (it helps to zoom in on the image). The faster shutter speed with a higher ISO value can minimize or remove motion blur, but if the ISO is too high there will be more noise in the image. It can also be overexposed if the ISO is too high when the location has abundant light.

For landscape images, use a tripod and shoot at low ISO for most images. This requires a large f/stop with smaller aperture. This is especially true when shooting outdoors, where there is plenty of light available. On overcast days, it may be necessary to increase the ISO to lighten up the shadows and bring out the details in the highlights. This depends on the photographer, because if the concept is to shoot in black-and-white, more contrast with dark and light may not require the detailed look in colored images.

High ISO Images

The general rule, based on what we know about ISO, is try to keep the ISO low or else the image will have too much noise. However, is that really true? Do photographers actually never use higher ISO when capturing images? Then how come many flagship cameras have ISO as high as 32000?

The Canon 5D Mark IV is an example of a premium DSLR camera that is capable of shooting well beyond ISO 1000. You might be wondering, well doesn’t that create too much noise? It can actually go up to ISO 32000 and even expandable to 50–102400. With these features, low light photography will benefit tremendously. That is the main reason for using high ISO in the first place.

The key here is that camera makers like Canon, Sony and Nikon are using microprocessor chips that contain noise reduction functions. These chips provide a higher SNR with more advanced noise reduction algorithms that allows using higher ISO. This breaks the rules of some photographers who advise us to keep ISO below 1000, because now there are camera models that well exceed this threshold. Camera ISO has pushed higher due to these developments.

Another reason to use high ISO, from a more creative standpoint and not technical, is to get a brighter and sharper image which captures the details. Although you get grain, it is not always unwanted. From an aesthetic perspective, it appears like a genuine photograph from the film days. As long as the composition conveys the image in good lighting, then there is nothing wrong with more grain. As long as the image can be recognized and appear impressionable, then there is no rule to say you cannot use higher ISO values.

ISO And Dynamic Range

With dynamic range, we are looking at where the noise floor meets with the maximum signal ceiling. The range refers to the differences in the darkest and lightest tones in an image in increments called stops. Increasing the ISO, increases what is called the gain or amplification of a signal that returns to what is called the middle gray area. The ceiling is the point at where the signal becomes completely saturated. As you increase the number of stops, you are doubling the brightness level.

In relation to ISO, the higher the ISO, the more brightness but the lower the dynamic range. The amplification performed leads to blown highlights in the image to pure white and it loses details. Without increasing ISO, typical digital cameras can capture up to 14 stops of dynamic range. In post, plenty of detail can be retrieved from the image with less noise when saved in RAW format.

ISO And Noise, Another Viewpoint

Noise is always created in the absence of light. In can only be reduced, but not totally eliminated. More detailed images require a balance in the dynamic range to show accuracy in detail (e.g. color, contrast in tones, brightness). When shooting in extreme darkness or when attempting to use long exposures (e.g. astrophotography), high ISO and noise is not really the issue when noise reduction is in use. Astrophotographers will even tell you that for their work, higher ISO actually decreases the visible noise. This is because it depends on what you are exposing and the light you have available.

Noise may not be caused by higher ISO after all (link). Does this require having to relearn what we were taught about higher ISO causes more noise? Actually, from the explanations I have read it does make sense. The premise is that noise is already present in an image in the absence of light and other factors (e.g. sensor noise, etc.). Therefore increasing ISO is not the direct cause of it. Since ISO brightens up the image, it makes the noise more visible. More importantly, a good exposure is one that was shot with the least noise due to good lighting.

The main cause of noise is poor exposure. This is due to the reduction if light or signal to the sensor, leading to a lower SNR. When bumping up the ISO, it brings out the noise that was a result of the poor exposure due to the amplification at the photosite. On the other hand, a good exposure will yield the least amount of noise even when bumping the ISO up. Even though it also bumps up the highlights, it brings out more details in the shadows. Getting a good exposure is a skill a photographer acquires and not really something that can be performed just by knowing how to do it.

ISO Invariance

It is hard to accept new concepts once you have ingrained the fundamentals in your head. ISO invariance is one of those concepts that makes things even more confusing (beginners be warned). It sounds counter-intuitive to conventional photography. This is because with new camera models, you can actually go ISO-less. That means ISO matters less in the exposure, which to many traditional photographers doesn’t make sense. It is time to look deeper as to why.

You may notice when using some of the latest digital camera models, you can take the wrong ISO setting but correct it later in post using a tool like Lightroom. It seems that ISO doesn’t really matter since you can always correct brightness in post. The reason is that these cameras produce the same image quality at the base ISO value, like the image was shot using the ISO equivalent from another camera.

According to Photography Life:

“If your camera sensor is perfectly ISO invariant, there is no penalty in noise if you brighten a photo in post-production rather than increasing your ISO in-camera.

Whether you set ISO 6400 in your camera and brighten it in Lightroom, or you set ISO 25600 in camera, you’ll end up with two files that have exactly the same amount of noise (though, again, the ISO 6400 photo may have more latitude in highlight recovery than the simulated ISO 25600).”

It really depends upon the camera model. Older DSLR are certainly not ISO invariant, so this concept does not apply to those camera models. It is best to check the camera maker’s documentation to verify if the sensor has ISO invariance features. That means if the camera is ISO invariant, the photographer can leave the ISO setting at base and create the exposure on just the aperture and shutter speed. Some cameras will be ISO-less up to a certain value from the base (e.g. ISO 100 to ISO 3200) or become ISO-less after a certain value (e.g. > ISO 1200).

Still confused? To explain further, ISO-less means that the brightness will take place not in-camera, but in post using software. Instead of analog amplification at the camera’s sensor, the brightening of the image is done using digital amplification in an image editing program like Photoshop or Lightroom. Sensors today have good dynamic range with noise reduction features that can really capture the image. It will still depend on how well the photographer captured the image i.e. best aperture and shutter speed combination. It will still be noisy if the light is poor, and no magic in post can really reclaim lost details from shadows if the exposure was not able to capture it.

Final Thoughts

Some might agree, others will disagree on technicalities or even the terminologies. I noticed that there are differences in perspective not only among the technically inclined photographers, but also from those photographers with a more artistic or creative background. The technical photographer may focus more on the gear or hardware, but the artistic photographer will focus on how to create the image.

From the technical perspective, the sensor always maintains the same level of sensitivity. It doesn’t magically change or alter what has been captured. It only modifies the level of amplification needed as set by the photographer with the ISO value. The sensor is collecting light for creating the image. Its collection of light depends on the the time the aperture is open and the shutter speed’s setting. Once the exposure button is released it records the light and captures and creates the image. The ISO can brighten the image, and that information is recorded along with other details in the RAW image. This can then be retrieved later in software.

It is ok to increase your ISO, but ideally only when there is not enough light. The problem with higher ISO is an increase in noise or grain. This can cause more blurry or softer looking images which lack contrast and details. Higher ISO settings are actually much more advisable for use with premium digital cameras that have high quality sensor features. Sensor manufacturers improve quality by developing higher SNR chips. These sensors have features that include advanced built-in noise reduction.

Exposure is created by the aperture and shutter speed combination, while ISO is used to determine the brightness of the image. With ISO invariance it seems that ISO is irrelevant to the exposure. Technicalities aside, the creatives don’t strictly follow ISO settings as there are ways to use high ISO for the type of look a photographer is going for. That is done even if there is already enough light for the sensor, so it is a part of the creative process.

The question of ISO and exposure remains contentious, but hopefully we can agree that it has to do with determining how bright the final image will look. I don’t think we should stop teaching about the exposure triangle, because that is a good starting point for beginners. It is good to learn the basics first, and then join in on the ISO conversation once exposure is fully understood.

I’d like to open the floor on the topic of ISO and exposure. Perhaps I missed some technicalities or totally did not get the point. Let me know if you have any thoughts on this by writing your comment below.