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The goal of this photography fundamentals article is to answer the following questions: What does ISO mean and how do ISO camera settings affect the way we capture photos?
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!
ISO is not the “International Standards Organization”
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.
To understand what ISO means in the world of digital cameras, it is helpful to look at film 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.
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 was progressed to the digital photography even though there is no such a 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 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.
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 film has grain 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.
ETTR is a digital phenomenon because at higher ISO, digital noise occurs in the shadows and not in the highlights. The goal is to slightly overexpose the image to reduce, minimize, or eliminate the noise factor in the shadows. The final exposure adjustments happen during the post processing.
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
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 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.
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.
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, it getting darker every minute and the shutter speed is becoming dangerously slow. This is when ISO settings come to the rescue.
By increasing your ISO value to ISO 3200, you tell camera to increase brightness of the image, after it was taken, by factor of 5. This means that higher shutter speed (5 stops higher) can be used and you can still capture an accurate exposure of the sunset scene.
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 there is plenty of sunlight 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.
Different Approaches to the Same Scene
Photography is all about recording light. The ISO functionality of camera is a key element in the photographic process. The examples above illustrate two different approaches of dealing with various light conditions. It is important to remember that one can approach the same scene in different ways, depending on what you want to achieve and what tools you have available to you.
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.
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.
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 the vision you have of your final photo with the scene in front of you and the light conditions.