Like many "quick questions" I’m asked, this wasn’t exactly something I could answer quickly. But to get him up and working, I explained to him that gain and ISO in the F3 are directly connected (where +6 dB equals two times the ISO), and that Exposure Index is more like a color correction made in post. This was a fine answer with regard to the F3, but most other cameras, especially RAW-recording cameras, work quite differently.This column is about the relationship between ISO, Exposure Index and gain, and how different cameras handle them. We’ll also talk a bit about white balance, as well. To start, let’s get some of these fiddly terms out of the way.
The ISO (International Standards Organization) method for rating film sensitivity has been the standard since 1974, when it combined the ASA and DIN film speed standards. We still hear people referring to film sensitivities in terms of ASA, but ISO is the real standard these days and uses the same basic scale. ISO is a logarithmic scale, where a doubling of the ISO value represents a doubling of brightness. For example, ISO 800 is twice as bright as ISO 400. This matches the same logarithmic scale of optical stops of light, where one stop equals a doubling of light. This makes the math of cinematography pretty comprehensible; I can double my exposure by opening my lens iris by one stop or by using film that’s two times more sensitive. A film stock’s ISO is determined by plotting its optical density versus exposure. While that sounds complicated, it’s a fairly straightforward and repeatable procedure. In the digital world, however, things aren’t so clear-cut.
Today, many digital cameras display their sensors’ sensitivity in ISO ratings. This is a great thing for cinematographers accustomed to that simple exposure math, and the logarithmic scale still applies as it did in film. Yet the process of rating a sensor’s ISO is quite a bit different from film. In 2006, the ISO standards for rating a digital sensor were broadened to five methods. Some are based on noise levels and image brightness, while others are based on comparisons to film stock. One method, the Recommended Exposure Index, allows for a fairly arbitrary declaration of ISO based on what a manufacturer regards as a properly exposed image. So, it’s fair to say that ISO is a loose standard for digital cameras, and it’s important to do your own camera sensitivity ratings when using a camera for the first time, just like cinematographers did before using a new film stock.
So, if ISO is our standard, why do we sometimes see it listed as an Exposure Index?