What is ISO?

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For anyone starting out with the basics of photography, one of the first things they learn is the ‘holy trinity’ of exposure: Shutter Speed, Aperture, and ISO. 

The first of these three, Shutter Speed, is reasonably straightforward. It tells us how quickly the shutter opens and closes, allowing us to adjust whether we take a photograph that captures the subject as frozen in time or in a more fluid manner.

Learning the aperture setting can be more challenging to understand at first. At its most basic, the aperture setting determines the size of the hole the camera uses to let light into the light-sensitive material (either via a digital sensor or film). The size of the hole allows us to adjust the amount of light that enters, but also allows us to change the depth of focus and helps us achieve the blurry-background effect known as bokeh.

But of the three of this trinity, ISO is something that can take a longer time to understand and appreciate fully. Many photographers in the beginning stages of their journey will just accept that ISO is what determines how much light is needed when shooting – but never stop to understand what film speed and ISO were and how they work.

What is ISO in Photography?

The term ISO—pronounced EYE-SOH—actually refers to the International Organization for Standards, which publishes voluntary guidelines and technical specifications for various physical and digital standards for industries such as healthcare, automotive, technology, and more. Though many believe that ISO is an acronym, this is technically not true. The name derives from the Greek word isos [ίσος], meaning “equal.” 

Though it is not the only system for measuring photographic sensitivity or ‘speed,’ most modern cameras and film use ISO.

In general, the ISO setting or film ‘speed’ in photography determines the sensitivity to light. The higher the ISO, the more light will be captured in the same amount of time. According to the ISO specifications, each time we double the number of our ISO settings, we increase exposure by one ‘stop,’ and the sensitivity to light is doubled. Therefore, when we go from an ISO of 100 to an ISO of 200, we can capture twice as much light as before. To double the sensitivity again, we must go from ISO 200 to ISO 400, and so on.

Using a higher ISO to increase light sensitivity, we can be more flexible with our other settings. Increasing the sensitivity means we can use a faster shutter speed or a smaller aperture (higher aperture number) with the same amount of light, allowing us to freeze motion or obtain a larger depth of focus than we might otherwise be able to achieve.

Example: If we can have a perfect exposure for a scene with our camera set to have an aperture of f8.0, a shutter speed of 1/25s, and ISO 400, but we need to shoot with a faster shutter speed because the branches in the trees are moving due to high winds, we can increase the ISO to 1600 (doubled twice, therefore increasing the exposure by two stops) and we now can increase the shutter speed by two stops to 1/100s and capture an image with much less movement.

However, this increased sensitivity does not come without a cost. Increasing ISO also means increasing the noise, or grain, visible in the photograph. To understand why increasing ISO results in this grainy effect, we need to look at how ISO works in practice. Let’s start by looking at how ISO works in standard film.

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How does ISO work in Film?

Photographic film is typically made up of a thin sheet of plastic with layers of tiny dots of light-sensitive materials, such as silver halide, applied to it. For black and white film, there may be only one layer of this material that darkens in the presence of light. Color film uses a similar system, but instead of a single layer of material, it will have multiple layers that react to different colors, or wavelengths, of light. 

Smaller dots of light-sensitive material result in film that is less sensitive to light (lower ISO numbers), but product photographs with finer detail and less grain. Films with larger dots, on the other hand, will be much more sensitive to light (higher ISO numbers), but these dots are much more noticeable and will result in photographs with more apparent grain.

But increased grain isn’t the only thing that occurs in higher ISO film. As Andy Grundberg wrote in his 1988 article Camera; On Choosing Film for the New York Times, as the increase film speed, “[t]he size of the grains in the film's silver-laden emulsion tends to get bigger, sometimes masking fine details. The purity and strength of the colors tends to diminish. And contrast, that nemesis of all photographers, rises.”


How does ISO work in Digital Cameras?

For digital cameras, things are a little different. As opposed to film, where the ISO rating affects the physical size of the light-sensitive dots, with digital cameras, we don’t actually change the size of the dots—in this case, the pixels on the sensor—when we adjust the ISO dial. 

In fact, we aren’t even changing the sensitivity at all. Though it may sound strange to hear, the digital sensor inside your camera has a fixed sensitivity to light. When we press the button to take a photograph, whether the ISO is set to 100 or 4,560,000, the number of light particles, or photons, that the camera senses is the same. 

So, if we aren’t changing the size and we aren’t changing the sensitivity, what exactly is happening?

The short answer is that we are changing the threshold for when the sensor records that it has sensed light. 

Imagine for a moment that you are sitting in a room, and your job is to open the door whenever you hear a knock. It’s very cold outside, and we don’t want to let the heat out, so I tell you to open the door only when you are 100% sure that you hear a knock. You’d sit quietly, waiting until you heard a knock. Maybe you heard something that sounds like a knock, but you aren’t sure, so you don’t open. 

Now imagine that I have a very important delivery coming and I don’t want to miss it. Now I tell you to open the door for anything you think might be a knock. Maybe you open the door, and the delivery person is there with the package, but perhaps it was a squirrel crawling across the roof. 

The sensitivity of your hearing hasn’t changed, but the threshold for what is required to open the door has changed dramatically.

Digital ISO works similarly. Although the sensitivity does not change, as we increase the ISO setting, we are telling the sensor that we don’t want to miss any light and to tell us when it thinks there was light. Maybe we get some false positives, but we are sure to capture all of the light that comes in. 

These false positives are what cause the grain, or noise, we see in photographs taken at higher ISO settings in digital photography. 

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Closing Thoughts and Points to Consider

Pros of Digital ISO:
Though film and digital ISO behave quite similarly in most situations, the physical nature of film’s light-sensitive material means that its behavior changes in certain conditions, such as extremely fast (<1/8,000s) or slow (>1/2s) shutter speed. Digital sensors are able to maintain performance far outside these ranges.

Pros of Film ISO:
While both film and digital photographs taken at higher ISO will have more ‘grain,’ the grain produced by film photography is often described as more natural and pleasant than the noise found in digital photography.

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