I BEFORE E
In film, Exposure Index (or EI) refers to an ISO rating used in a particular shooting situation that’s different from the actual film speed rating. That means, in some situations, you may rate a film speed faster (higher ISO) or slower (lower ISO) to get a desired effect. For example, if I had ISO 400 film stock, I could rate it as 800, knowing that in post I would have to push it to make the image brighter. The term "push" here means that I would brighten the image through a post process (film scan or otherwise) to make it 800. You may see this simply written as EI 800. The important consideration is that EI implies that a post adjustment is used to get the image to a correct exposure, which could result in some image issues such as increased noise.
Digital cinema cameras, such as the ARRI ALEXA and Sony F3, give you the option of setting EI in-camera. Just like film, EI implies that a post process is going to be applied to the image to change the resulting exposure, but the difference here is that we’re seeing the results immediately. In the case of the ARRI ALEXA, the sensor has a native rating of 800, and any adjustments beyond that are considered EI changes. The sensor is like a set film stock, and pushes and pulls to that stock are done digitally in-camera to give you a result. Push too high, to ISO 3200, for example, and the image will become noisy. The sensor becomes like a film stock in many ways and, if you capture the RAW data from it, you would see that the ISO is always fixed at 800 and all EI adjustments are done in post. Because the sensor itself is unchanged, the dynamic range above or below the middle gray point remains fixed, so adjusting EI also changes your stop range.
ARRI made this chart to help explain the process [Figure 1]. Notice that at ISO 800, you have 7.4 stops up and 6.6 stops down, but it changes as EI is adjusted digitally. RED and other RAW-recording cameras work very much the same way, and we’ll talk further about that in a minute.
Going back to the Sony F3, it has an EI mode in-camera that fixes the sensor at 800, but allows you to see the EI changes on the monitor output. The idea here is that you can record the S-Log output and do the EI changes in post, just like you would do with RAW data. Here’s Sony’s chart of what happens when you change EI in post [Figure 2].
So, you can think of EI as sort of a virtual ISO change; the sensor itself always has the same sensitivity and dynamic range, and we just mess around with it in post to get the results that we want.