What exactly is ISO? A Step-by-Step Guide to Manual Mode for Beginners

Highlights: Higher ISOs allow you to shoot in darker or dimmer conditions, but they add grain or noise to your images.

ISO is a measure of light sensitivity. In photography and filmmaking, it is part of the exposure triangle. Knowing what it is, how it works, and how it affects your work is critical to obtaining the best possible exposure in any shooting situation.

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What exactly is ISO? A Step-by-Step Guide to Manual Mode for Beginners

What exactly is ISO?

ISO is the industry standard for determining the light sensitivity of film stock. It is also known as “film speed,” with ISO 100 being slower than ISO 800. Film speed should not be confused with shutter speed or frame rates. They are all distinct entities. Slower film speeds indicate that the film is less sensitive to light and thus requires more exposure to create an image.

ISO 100 is regarded as the “base ISO.” When you increase your ISO to 200, you gain about another stop of light and effectively double the amount of light reaching the sensor. ISO 400 is two stops brighter, or four times brighter, than ISO 100. By increasing the ISO to 1600, you can increase the brightness by four stops.

The increased sensitivity to light caused by faster film speeds, on the other hand, will result in grainier-looking images. Lower speed films have smaller photosensitive grains that are less sensitive to light; higher speed films have larger grains that can pick up more light. However, because these grains are larger, they are more visible in the final image.

ISO stands for sensitivity in photography and filmmaking. A higher ISO means more sensitivity, but it also means more grain.

Why is ISO called ISO?

The letters I, S, and O in “ISO” are unrelated to light, film chemicals, or optics. They are referring to ISO 12232:2006 (it was previously ISO 5800:2001 until it was updated in 2006), the International Organization for Standardization’s yardstick for color-negative film

The International Organization for Standardization, based in Geneva, is in charge of developing and publishing commercial, industrial, and technical standards. Each of these standards is assigned a number that is preceded by ISO. 

There are many other ISOs in the world, such as those for vehicle roof load carriers, but ISO 6:1993 refers to black and white negative film and ISO 2240:2003 is the standard for color reversal film.

ISO vs. ASA 

ISO vs. ASA? There is no distinction. The acronym ASA stands for American Standards Association, but the measurement is the same. ISO 200 film is equivalent to ASA 200 film. However, ISO has been the standard term since 1974.

In a digital world, ISO

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Even though ISO refers to film speed and is deeply rooted in analogue photography and filmmaking, its name is still used as a measurement of light sensitivity in digital cameras. The ISO setting on your digital camera is part of the exposure triangle, along with the aperture and shutter speed that you use to capture an image. When you set your DSLR or mirrorless camera to ISO 100, its sensitivity to light should be the same as ISO 100 film.

This increased sensitiveness is achieved by increasing the voltage and boosting the signal to the sensor in your camera. This, like turning up the volume on a staticky radio broadcast, amplifies everything the sensor sends to the processor, including the interference. Digital ISO produces noise in the same way that film ISO produces grain.

How to Select an ISO

While there is still an exposure triangle in filmmaking, one of its key features has been fixed. Your shutter speed will be determined by your frame rate and thus cannot be changed. It is therefore critical to understand how to choose the best aperture and ISO for your desired exposure. 

Digital cameras are capable of producing extremely high ISOs, but the trade-off is noise. As a general rule, you should shoot at the lowest ISO possible. 

Assume you’re shooting at 30 frames per second, which corresponds to a shutter speed of 1/60 second:

  • For good exposure outside on a sunny day, use ISO 100 or 200. If it’s very cloudy, you might need to turn it up a notch.
  • A well-lit interior will necessitate an ISO of around 800. 
  • For a dimly lit interior, try ISO 1600.

Of course, if you’re using a very wide or very narrow aperture, you may need to adjust your ISO to compensate. If you find yourself shooting in a very bright situation, with your ISO set as low as it can go and your aperture set as it should be, but your scene is still over-exposed, consider using an ND filter to help block some of the light.

Dual native ISO and native ISO

The native ISO of your camera is the highest ISO that does not require a change in voltage to the sensor and can produce the best picture quality. Because nothing needs to be amplified, the ISO has the lowest signal-to-noise ratio. Consider it your ideal ISO. It is not, however, always the base ISO. The Canon EOS R6, for example, has a native ISO of 800, making it a good interior and low-light camera. 

A dual native ISO camera has two optimal native ISO settings, usually one low and one higher. This gives you a lot of flexibility when shooting in bright light or low light.

1024px Iso comparison

What is the gain?

The signal to your sensor is amplified by digital ISO. More voltage equals greater brightness. The amount by which that signal is amplified is referred to as gain. Decibels are used to measure gain. Some digital cameras let you adjust your gain rather than your ISO, but the effect is the same: they change your camera’s sensitivity to light.

What is EI?

EI is an abbreviation for exposure index. But what exactly is the exposure index, and how does it relate to sensitivity? When you increase the gain, you are effectively increasing the signal to the sensor to increase sensitivity.

When you change the exposure index, you change the sensitivity between the sensor and the processor. This is more like a post-processing adjustment–something done with film when you “push” it during processing–but the result is instant. As a result, increasing the exposure index will increase your sensitivity.

How do gain and EI stack up?

It’s probably a good idea to think of adjusting gain as akin to changing your ISO with different film stock. The exposure index is more akin to making changes to a film after it has been shot. Both will help you control your exposure; it’s just a matter of which one you prefer.

ISO’s Prospects

One of the most significant advances in digital photography and filmmaking has been the improvement in sensitivity and the ability to control noise. I’m not sure how long we’ll continue to refer to ISO, but it’s still here and serving a vital purpose.

Eager to learn more?

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About the author: Trent (IMDB Youtubehas spent 10+ years working on an assortment of film and television projects. He writes about his experiences to help (and amuse) others. If he’s not working, he’s either traveling, reading or writing about travel/film, or planning travel/film projects.

What exactly is ISO? A Step-by-Step Guide to Manual Mode for Beginners

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