ISO in Photography – Breaking Away from the Exposure Triangle

This photography fundamentals article aims to answer the following questions: What does ISO in photography mean, and how do ISO camera settings affect how we capture photos?

ISO in Photography – Breaking Away from the Exposure Triangle

ISO is one of the most misunderstood concepts in digital photography. If you think ISO is an abbreviation for “International Standards Organization” or that it is part of the exposure triangle, then you definitely need to read this article from beginning to end!

What is ISO in Photography Mean?

To put the myth to rest once and for all, ISO is not an acronym for the “International Standards Organization.”

Here is the quote from the International Organization for Standardization website:

Because ‘International Organization for Standardization’ would have different acronyms in different languages (IOS in English, OIN in French for Organisation internationale de normalisation), our founders decided to give it the short form ISO. ISO is derived from the Greek isos, meaning equal. Whatever the country, whatever the language, we are always ISO.

It is helpful to look at film cameras to understand what ISO means in the world of digital cameras.

ISO in Film Photography

The film cameras record light onto a film emulsion that photographers need to physically load into the camera.

Each roll of film has a film sensitivity rating that is expressed as a number

ISO 100, ISO 200, ISO 400, ISO 800, ISO 1600, and so on

Each number represents the sensitivity of the film to light. Lower ISO numbers mean that the film is not as sensitive to light, and higher numbers mean that the film is more sensitive to light.

The plastic emulsion of camera film is coated in delicate light-sensitive layers of silver halide crystals. Film emulsion that is rated as “highly sensitive to light” has layers of crystals that are coarser and larger. The larger grains allow the emulsion to record light more quickly. Likewise, film emulsion that is rated as having a lower sensitivity to light has a finer film grain.

Related: How To Make Photos Look Like Film In Lightroom

The increase in size and coarseness of the film’s silver halide crystals results in an increase in the light sensitivity and the ISO speed (another term used in film). It results in pictures with more grains and lesser saturation, detail, and dynamic range.

ISO in Digital Photography

ISO speed, as a term, progressed to digital photography even though there is no such thing as “sensor sensitivity.”

ISO in digital photography means something completely different – applied gain.

Gain signifies the increase in the image’s brightness after capture.

Let me explain.

The camera sensors consist of tiny units known as pixels. When we read the specifications of a camera, it may say that it has a 20 Mpix sensor, which means that it contains 20 million tiny units or pixels.

Related: Glossy vs Matte: Understanding the Difference

When we press the shutter and light hits the sensor, every pixel measures light intensity by counting the number of photons reaching the pixel. The charge of the photons changes the voltage in each pixel; the voltage values are recorded by the camera. The RAW data is a collection of recorded voltage values from all 20 million pixels. The amplification of the signal happens after the data is collected by applying the gain.

For example, when you set the ISO to a value of 200, the camera still records the image at a base ISO of 100. When the image is recorded, the gain is applied to boost the brightness by a factor of two.

The ISO increase occurs at the cost of details, sharpness, and dynamic range. This part of ISO remains constant in both film and digital photography because increasing the ISO has always come at a cost.

ISO in Photography – Breaking Away from the Exposure Triangle 1

Another major difference between the digital and film worlds, when shooting at higher ISO values, is the grain and noise. Film photography has grain; digital photography has noise. While the film has grain in the highlights, digital has noise in the shadows. One of the major aspects determining how we expose an image in the digital world is ETTR.

ETTR (Expose to the Right) vs ETTL (Expose to the Left)

ETTR refers to “Expose to the Right,” and ETTL refers to “Expose to the Left.” ETTR means slightly overexposing the image, and ETTL means slightly underexposing the image.

ETTR is a digital phenomenon because, at higher ISO, digital noise occurs in the shadows and not in the highlights. The goal is slightly overexpose the image to reduce, minimize, or eliminate the noise factor in the shadows. The final exposure adjustments happen during the post-processing.

ISO in Photography – Breaking Away from the Exposure Triangle 2

The image was exposed to the LEFT. The sky is well exposed, but the shadow areas are underexposed.

ISO in Photography – Breaking Away from the Exposure Triangle 3

The image was exposed to the RIGHT. The shadow areas are well exposed, but the sky is overexposed but not “clipped.”

ISO in Photography – Breaking Away from the Exposure Triangle 4

The final processed image with balanced shadows and highlights.

Using ISO to Control the Aperture and the Shutter Speed

While the shutter speed controls the duration of time that light reaches the sensor, the aperture controls the total amount of light that reaches the sensor; the ISO does not control the light.

You can, however, affect the aperture and the shutter speed for a particular shot using ISO.

Shooting wildlife requires consistently high shutter speeds. Sometimes, when shooting in shades or during sunrise and sunset, the light cannot provide sufficient exposure. Slowing the shutter speed introduces motion blur, which spoils the shot unless a motion blur is the desired effect. Instead, we can easily increase the ISO and keep the desired shutter speed without compromising the image we envision.

Similarly, while shooting Milky Way, getting a perfect shot at a low ISO value isn’t always easier. The star trails are introduced into Milky Way photography if we use the shatter speed value above 25-30sec. To compensate, a high ISO can be used.

ISO in Photography – Breaking Away from the Exposure Triangle 6
The ISO 5000 and the Shutter Speed of 20 sec were used.

How to Use ISO Effectively On Your Digital Camera

If you take out your digital camera and switch it on, you will notice that the LCD screen displays the current ISO setting. On many cameras, it is possible to change the ISO number through the simultaneous operation of a function button and a command dial. You will notice that you can quickly scroll through the ISO settings in this way.

What does all of this mean for your photography in practical terms? Let’s look at a few real world examples of ISO in use.

Photographing a Beautiful Sunset

Let’s imagine that you are admiring a gorgeous sunset at the beach, you have your camera with you set to Aperture Priority, and you are shooting hand-held. You’d like to record the scene, but the sun is almost below the horizon, getting darker every minute, and the shutter speed is dangerously slow. This is when ISO settings come to the rescue.

By increasing your ISO value to ISO 3200, you tell the camera to increase the image’s brightness after taking it by a factor of 5. This means that a higher shutter speed (5 stops higher) can be used, and you can still capture an accurate exposure of the sunset scene.

ISO in Photography – Breaking Away from the Exposure Triangle 7
For example, before shooting a winter landscape, I had to boost the ISO to the value of 400.
It was getting darker, and I was shooting hand-held without a tripod.

Photographing a Landscape on a Sunny Day

Let’s imagine that you are out on a bright and sunny day in the countryside. You stop on the side of the road and see beautiful rolling hills adorned with flowers in front of you.

You have your camera with you, and it is set to Aperture Priority once more. Because plenty of sunlight is available to light the scene, you understand intuitively that you can use an ISO setting of 100 on your camera. And as a result, take advantage of a shutter speed setting that is fast enough to combat the erratic breeze that is blowing at your back. The abundance of sunlight also means that you can use a narrower aperture of f8 to ensure that everything in the scene is sharp and in focus.

ISO in Photography – Breaking Away from the Exposure Triangle 8
Since I was shooting in broad daylight I did not have to worry about ISO. I kept it at 100.

Different Approaches to the Same Scene

Photography is all about recording light. The ISO functionality of the camera is a key element in the photographic process. The examples above illustrate two different approaches to dealing with various light conditions. It is important to remember that one can approach the same scene differently, depending on what you want to achieve and what tools you have available.

For example, if the photographer had a solid tripod during that gorgeous sunset, he or she could actually have used a much lower ISO setting. By stabilizing the camera on the tripod rather than hand-holding it, it would be possible to use the base ISO setting of 100 rather than 3200.

ISO in Photography – Breaking Away from the Exposure Triangle 9
Since I was using a tripod here and I wanted to achieve long exposure effect in the water, I kept ISO at 100 which allowed me to get a 4 seconds Shutter Speed.

A dark scene and low ISO would result in a slower shutter speed so that enough light could be recorded for accurate exposure. But if the camera had been stabilized on a tripod, there would be no camera shake and vibration to deal with. The result would have been a photo with less noise, more saturated color, and greater levels of detail.

Important: By practicing photography on a regular basis, one develops a more sophisticated understanding of the dynamic relationship between ISO, aperture and shutter speed, and how these variables combine to produce a photo.

ISO in Photography | Conclusion

While it is established that ISO is not part of the exposure triangle in digital photography, it still affects the overall brightness of the image and helps us control the parameters (aperture and shutter speed) that directly affect the exposure.

By developing an understanding of these dynamic relationships, you will also be able to more accurately match your vision of your final photo with the scene in front of you and the light conditions.

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25 thoughts on “ISO in Photography – Breaking Away from the Exposure Triangle”

  1. Thanks for this article, ETTL has almost always been my inclination, not anymore !

  2. Your definition of ISO is a delayed April Fool’s Joke, right?

    “ISO” is, in fact, an accepted acronym for International Organization for Standardization. While the actual origin of the acronym may be as you say, you need to tell the whole story.

    It’s use in film speed designations comes from subsuming the older ASA (American Standards Association) film speed designations in 1987 (documented in ISO Standard 5800:2001, first published in 1979, updated in 1987, and also subsuming the German DIN film speed standard). Anyone who shot film in the last century will definitely remember ASA designations for Kodak and Japanese films like Fuji, and DIN designations for European films like Agfa.

    ISO 5800:2001 defines film speeds for colour negative films specifically. There are a couple of other related standards: ISO 6:1993 for B&W negative films, and ISO 2240:2003 for colour slide film. They are all more or less on the same scale.

    The ISO designation for digital photography should have absolutely been ported from the film era … because it helped those of us who shot film to make the conversions in our heads … shooting ISO 100 in digital gave similar camera settings to shooting ISO 100 in film. Old rules of thumb, like the Sunny 16 Rule, still made sense for us old shooters. And I have heard more complaints about aperture numbers than I have ever heard about ISO numbers – they actually make some sense at first glance.

    The rest of your explanation is more or less correct. Increasing the ISO setting on a DSLR does not make the sensor more sensitive as is the popular belief … it determines how much a captured image is amplified by the electronics in the camera to create the image, similar to an audio amplifier. As such, the more it is amplified, the more it is subject to having digital noise injected into the image, particularly in the lower tones.

    Anyway … I felt that a corrected explanation of what ISO stands for was necessary … there is just too much folklore floating around, and as an educator, it is easier to correct those at the source.

    • Because ‘International Organization for Standardization’ would have different acronyms in different languages (IOS in English, OIN in French for Organisation internationale de normalisation), our founders decided to give it the short form ISO. ISO is derived from the Greek isos, meaning equal. Whatever the country, whatever the language, we are always ISO.

      • As I indicated … it is an accepted acronym for International Organization for Standards. Please tell the whole story so the rest of us don’t have to correct it.

    • PLEASE! ISO is short for ISOS meaning equal. It is NOT anything to do with international standards. You probably think ultimate means best too when it actually means LAST! Nothing more than that.

  3. This is a great article. It explains so much about how ISO and shutter speed and aperture are related.

  4. Hi Viktor,
    Have to say I agree with Marty here!
    But I didn’t quite understand your statement that ISO was not part of the ‘Exposure Triangle”. To me the three elements have always been shutter speed (mechanical or electronic equivalent), aperture and film speed (or in digital terms, gain)?

    • Tony, in digital photography there is no such a thing as “sensor sensitivity” as most people assume. The film sensitivity was translated to digital photography as GAIN. The gain happens after the shutter is closed after the signal was recorded. The gain is a post exposure process.

      As for the name ISO, it is not an acronym it is a made up company name like Pepsi.

      The goal of the article was to address 2 biggest misconceptions about ISO in digital photography, nothing else. But we do not have to worry about it, it will not affect the way we take pictures which is the most important part.

      • Sorry, you really need to learn to research before writing technical thoughts, try to Google things first.

        The ASA and DIN film speed standards have been combined into the ISO standards since 1974.
        Current system: ISO
        The ASA and DIN film speed standards have been combined into the ISO standards since 1974.
        The current International Standard for measuring the speed of color negative film is ISO 5800:2001[17] (first published in 1979, revised in November 1987) from the International Organization for Standardization (ISO). Related standards ISO 6:1993[15] (first published in 1974) and ISO 2240:2003[16] (first published in July 1982, revised in September 1994, and corrected in October 2003) define scales for speeds of black-and-white negative film and color reversal film, respectively.
        The determination of ISO speeds with digital still-cameras is described in ISO 12232:2006 (first published in August 1998, revised in April 2006, and corrected in October 2006).
        The ISO system defines both an arithmetic and a logarithmic scale.[36] The arithmetic ISO scale corresponds to the arithmetic ASA system
        The ISO 12232:2006 standard
        The ISO standard ISO 12232:2006[60] gives digital still camera manufacturers a choice of five different techniques for determining the exposure index rating at each sensitivity setting provided by a particular camera model. Three of the techniques in ISO 12232:2006 are carried over from the 1998 version of the standard, while two new techniques allowing for measurement of JPEG output files are introduced from CIPA DC-004.[61]

        The ASA (as for American Standards Association) photographic exposure system, originally defined in ASA Z38.2.1 (since 1943) and ASA PH2.5 (since 1954), together with the DIN system (DIN 4512 since 1934), became the basis for the ISO system (since 1974), currently used worldwide (ISO 6, ISO 2240, ISO 5800, ISO 12232).

        Film speed
        From Wikipedia, the free encyclopedia
        Film speed is the measure of a photographic film’s sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to describe the relationship between exposure and output image lightness in digital cameras.

        • Sorry Marty and Jerry but the article stated the facts about ISO exactly as you have. You added some interesting details but did not “correct ” anything. Thanks, Viktor, for writing the best article on ISO for digital photography I have ever read.

          • I know it is very controversial 🙂

    • I do not think you can. I believe you need to install a dedicated app to control ISO manually.

  5. Thanks, that’s a lot of additional knowledge in photography. As a summary: 1) ASA/DIN is for Film (chemical) Sensitivity or Speed for both USA and Europe. 2) ISO/OIN is for Digital Camera Sensor (CMOS – electronic) Sensitivity in Pixels for both USA and Europe.

  6. Thanks for this article. It was simplistic enough to help me with my settings understanding. I don’t need the history of ISO, like some of the other readers. Most of us are just trying to figure out how to get the best pictures.

    • Fantastic reply!!!. I was just trying to understand what the heck is ISO for. I don’t care about the history of ISO like other readers. Just my view. Thank you Viktor for your wonderful explanation. It helped me and I’m sure others as well.

  7. The standards are not my concern. Best take-eaway is to use TTR for digital, in poor light. Wish I knew this before!!
    Thanks V

  8. Extremely Helpful… And Easily Understood… For a Change…

  9. I accept the technical explanation of ISO as ‘digital gain’. The concept makes sense to me. But I would not extrapolate this to claim that it is not a part of the Exposure Triangle merely due to the timing of when the gain gets applied to the image. It is certainly a functional part of the exposure triangle as photographers think about and modify their exposures while shooting. On any modern mirrorless camera with EVF, modification of ISO enables the photographer to see the impact of ISO changes on the exposure BEFORE shooting the image, and ISO changes made on a standard DSLR are made before the shutter is pressed to a) directly modify the resulting exposure/image brightness, or b) to enable modification to shutter speed or aperture. Therefore, regardless of the semantics of calling it digital gain or ‘sensor sensitivity’ it is still a functional part of the exposure triangle with which photographers control image brightness, and is still an active element photographers include in their thinking process during shooting. The way you’ve described this, you might as well be dragging the exposure slider up during post-processing and I believe photographers integrate ISO into the creative side of their photography. In short, despite your explanation, this will not fundamentally alter how I manipulate aperture, shutter speed, or ISO during my own shooting. And since there are three elements to control, and since they impact the final exposure of an image, I will continue to call the collectively, the exposure triangle.

  10. Anybody know when the term “Exposure Triangle” was first used? By whom?

  11. So with Raw data, data that is captured by the sensor without any other settings, the ISO setting is applied after the capture and is embedded irreversibly in the Raw data. Is that a correct assumption?

    • that’s right, the camera always captures light at base ISO and if you shoot JPEG the gain applied in the camera processor and when you shoot RAW the ISO info is embedded into EXIF data and the gain is applied in Lightroom.


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