If you’re new to ISO, native ISO and base ISO, this is the video and article for you.
Start by watching the video:
What is ISO?
ISO is “part-acronym” (see below) for the International Organization for Standardization (informally International Standards Organization, or ISO). The organization is the body that standardizes things so people can compare different products or results.
When it comes to cinematography, ISO follows ASA, the film system of rating sensitivity.
The ISO rating in a camera tells you, at a glance, how sensitive the sensor is at any given time.
How do you write ISO numbers?
ISOs change in stops, just like the aperture an shutter, and the typical values are:
ISO 50, 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600 and so on.
ISO is not an absolute measure. What do I mean by that?
E.g., a kilometer should be the same length wherever you go, but ISOs need not agree between different cameras. Even within a camera, it might not agree depending on the profile you select!
In other words, the results of ISO 800 in one camera might not be the same as ISO 800 in another camera.
For similar gamma encoding and color space, though, it gives a standard measure of what to expect when you deviate from it.
Let’s assume we have two cameras both filming in strict Rec. 709. In this case the ISO values of both cameras should be close enough to let us judge exposure. This means, if you light for an aperture of T2.8 on camera A at ISO 800, camera B should give you a similar exposure at ISO 800.
It only works if the gamma encoding (log, Rec. 709, etc.) is similar on both cameras. The exposure might agree. However, the noise characteristics (see later) might not agree.
So, when we talk about ISO, it’s best to talk about what it does within a specific camera. When there are more than one type of camera involved, we need to test to understand the relationship.
What is native or base ISO?
Every sensor is made of pixels. Each pixel produces maximum dynamic range and color (in other words, its best performance) at only one ISO. This ISO is the camera’s native or base ISO.
The native ISO is chosen by engineers at camera companies based on the color science, sensor characteristics and goals of the camera. All we need to know is which number it is!
Always follow this rule:
If you want the best quality possible from a sensor, stick to its base or native ISO.
There are no exceptions to this rule.
This video and article will help you understand this better:
If there is only one best ISO, why do cameras have more than one ISO?
ISO is also a tool for exposure. You raise the ISO when you need more exposure, and you reduce the ISO to reduce light hitting the sensor.
This has drawbacks, of course. When you deviate from the base ISO, you will always reduce dynamic range and color performance. This is true of most mirrorless cameras.
However, on cameras like the Arri Alexa, you get similar dynamic range and colors over a wider ISO range, like 400 to 1600.
The two things that happen when you change ISO
For simplicity’s sake, you only need to worry about two things when you change ISO:
- Dynamic range, and
- Color Performance
Here’s a comparison between two ISOs on the Sony a7R II:
Note how the color has changed. If you need to cut together shots at different ISOs, you might need to color match and also reduce/add noise to get them to match.
Colorists face this all the time. They often get footage from cinematographers who ignore the basic workings of ISO.
Just because a camera allows you to raise ISO doesn’t mean you should! A good cinematographer will test a camera to understand what ISO range is best to film in. There’s always a compromise deviating from the base ISO.
Sometimes the differences are negligible, sometimes it isn’t. Only you can know what is right for your film.
Here’s how ISO works in the context of exposure:
ISO ‘Ranges’ or The ISO Window.
I use this term to denote the range of ISOs under which you get similar performance. You could also call it the ISO window.
Let’s assume you test a camera and you find a group of ISOs with relatively similar noise and color characteristics. When filming, you should stick within this range if you want to match footage easily in post.
Cameras that shoot in 8-bit 4:2:0 typically have very poor color already. It’s not easy to grade them to match, and the latitude is minimal.
All this applies to Log, RAW or any other profile. The closer the footage matches out of camera, the better it will be in the end.
However, there are always situations where you absolutely can’t stay within the ISO window. You ran late and the light is too low. You might have to bump up the ISO to get the shot.
Happens. You roll with the punches. This isn’t an exact science.
How is ISO pronounced?
This is controversial.
This is the text from the official ISO page:
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.
ISO.org
Important to note:
- ‘derived’. ISO is NOT iso. It is derived from iso. It is still ISO in all caps.
- ‘short form’. ‘Short form’ is a misleading term. E.g., tux is the short form of tuxedo. ISO is not the short form of any word in that context. The other way short form is used, is to mean either an abbreviation (doctor becomes Dr.) or an acronym (United States of America is USA). The abbreviation definition doesn’t work in the case of ISO, so the only other short form that applies is I.S.O., the acronym.
- In the English language, an acronym is usually pronounced in individual letters. USA is U.S.A. There are exceptions, like AIDS (these are called word acronyms). You could also have it both ways. ASAP can be pronounced as ‘ay-sap’ or A-S-A-P. The correct way to pronounce ISO as an acronym is I-S-O, and not ‘ai-so’ or ‘eye-so’.
- ISO as an organization hasn’t specified ISO is a word acronym. It was created by non-English speakers, which is why the confusion exists. They are to blame, not you or I.
- I am an engineer, and my professors always pronounced ISO as I-S-O, so there’s that. ISO is used for a lot of things, cameras are just a tiny part of it.
Which is why I call it I-S-O (eye. es. oh). You can agree or disagree, but that won’t change the way I pronounce it!
Anyway, the way you pronounce ISO has no bearing on your work. Call it whatever you want, and let others call it whatever they want.
I hope this article has shed some light on how ISO works in cinematography.
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