Working sensitivity rating for film — not the true ISO, but calculated for exposure metering. Pairs with light reading and aperture to set shutter.
You set the camera for a specific film stock—say, Kodak 400—but the lighting situation is borderline. You could expose the stock nominally at ISO 400, but risk underexposure in the edit. So you push the EI to 800 or 1600. This isn't the physical sensitivity of the film—that doesn't change. The EI is your calculated working value for setting the correct exposure in practice. You tell the light meter: Calculate for 1600, not 400. The meter will then give you different aperture and shutter values.
On set, it works like this: Measure the light value, factor in the EI on the secondary meter or in your head, then combine it with aperture and shutter. It's similar with modern digital cameras—you set the EI in the menu. However, analog camera operators with handheld light meters remain classic: The EI determines at which film sensitivity the device calculates. If you have to set a slow 50-speed emulsion with EI 100 (because you want to push it 2 stops), the meter reads accordingly—with all the consequences in the lab or scan.
The tricky part: EI and ISO are not identical. ISO is a standard. EI is your creative or forced decision. You can run a 100-speed film at EI 50 or EI 200. The former gives you more shadow detail and reduces grain—the film is underexposed, and the lab corrects it in development. The latter is a push—the film is overexposed, the highlights blow out, and the shadows crush. But sometimes you need exactly that contrast profile.
For the work, this means: Consistency is mandatory. If you meter at EI 500, that must apply to the entire scene—and the colorist/lab technician must know. In digital, the EI is more of a metering trigger, but even there—if you have to run with increased sensor gain, you have similar consequences: more noise, a flatter tone curve. The EI is your tool for correct exposure under unfavorable conditions, not for creativity without cost.