In this video and article we’ll cover what ISO is, and what happens when you change it. First, if you’re completely new to this, please watch my video on the exposure triangle first.

First, here’s the video:

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What is ISO?

ISO stands for International Organization for Standardization (informally International Standards Organization), and is the body that standardizes things so people can compare different products or results.

When it comes to photography, ISO follows ASA, the film system or rating sensitivity. In short, the ISO rating should tell you at a glance how sensitive a sensor is at any given time.

Note: They’re not an absolute measure. E.g., a meter should be the same length wherever you go, but ISOs need not agree between different cameras. But within a camera, it gives a standard measure of what to expect when you deviate from it. 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.

What is native or base ISO?

Every sensor is made of pixels. Each pixel produces maximum dynamic range and color (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:

If you want the best quality possible from a sensor, stick to its base or native ISO

There are no exceptions to this rule.

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 get footage from DPs who ignore the basic workings of ISO.

Just because a camera allows you to raise ISO doesn’t mean you should. In fact, there’s no instance where raising (or lowering) the ISO from its native ISO is good. However, there are acceptable levels of tolerance.

ISO ‘Ranges’

I use this term to denote the range of ISOs under which you get similar performance. In my Sony a7S II and a7R II guides, I have comprehensively covered this. The basic principle is you have groups of ISOs with similar noise and color characteristics, and you should stick within these ranges 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 color model. So the closer the footage matches out of camera, the easier it is for everyone concerned.

I hope this article has shed some light on how ISO works basically.

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