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’s part of the exposure triangle, then you definitely need to read this article from beginning to end!

Definition of ISO in Photography – Breaking Away from the Exposure Triangle

Unlike aperture and shutter speed, ISO doesn’t actually contribute to exposure when the image is recorded. 

Let’s understand ISO from another perspective. ISO is just like the amplification of sound. The more you amplify it, the louder the sound and the less clear it will be. It can eventually reach a point where it is just noise and the words and music are indistinguishable. To avoid that, there are ways of turning up the volume which, like photography, depends on equipment.

More...

ISO is not the “International Standards Organization”

To put the myth to rest once and for all, ISO is not an abbreviation for the “International Standards Organization.” The origin of ISO is actually a “word-forming element meaning ‘equal, similar, identical; isometric,’ from comb. form of Greek Isos ‘equal to, the same as’ (e.g. isometer ‘like one’s mother’).

ISO in Film Photography

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

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.

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 is a term that shouldn’t have progressed to the digital photography world because there is no such thing as digital sensor sensitivity. ISO in digital photography means something completely different—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 on a camera, it may say that it has a 20 Mpix sensor, which means that it contains 20 million tiny units or pixels.

When we press the shutter and light hits the sensor, every single pixel measures the intensity of light 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 was 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 the constant in both film and digital photography because increasing the ISO has always come at a cost. 

ISO is also now available at the change of a dial and today’s cameras are much better in terms of offering high ISO performance.

Another major difference between the digital and film worlds is the grain and noise. Film photography has grains; digital photography has noise. While film has grains in the highlights, digital has noise in the shadows. This is one of the major aspects that determine how we expose an image in the digital world - 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 to slightly overexpose the image and ETTL means to slightly underexpose the image.

ETTL was popular in the days of film photography for the simple reason that grains were prevalent in the highlights. Therefore, exposure was shifted slightly to the left (generally 1/4th to 1/3rd of a stop) to produce a cleaner image devoid of grains.

ETTR is a digital phenomenon because digital noise occurs in the shadows and not in the highlights. The images are now slightly overexposed to reduce, minimize, or eliminate the noise factor in the shadows.

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

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

The final processed image with the balanced shadows and highlights.

Using ISO to Control the Aperture and the Shutter Speed

The digital world of photography has opened up a variety of possibilities. Modern-day cameras are consistently breaking the barriers of low-noise performance, which aids both photography and cinematography. While the shutter speed controls the duration of time that light has to reach 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 is not always capable of providing 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.

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.

Similarly, while shooting Milky Way, it isn’t always easier to get a perfect shot at a low ISO value. 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.

The ISO 5000 and the Shutter Speed of 20 sec was used.

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. Some photographers are even creatively exhibiting their high ISO photography by using sensor noise as a tool!

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

  • Marty Mar says:

    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.

    • Viktor Elizarov says:

      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.

      https://www.iso.org/about-us.html

      • Marty Mar says:

        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.

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

  • 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)?

    • Viktor Elizarov says:

      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.

      • jerry+collins says:

        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.

        • Viktor Elizarov says:

          how is it relevant to my article?

  • >