Index

On set, you work with light and aperture—the index is your tool for quickly grasping this relationship. It indicates which aperture setting you need for a given film sensitivity to correctly expose a scene. Specifically: at ISO 100 and a certain exposure value (measured in EV), you would need, for example, f/2.8. If you increase the ISO to 400, the possible apertures shift up by two stops—now f/11 is sufficient for the same scene. This is the index in action.

The practical relevance lies in the speed of decision-making. You measure the light intensity with an incident or reflected light meter, and the index immediately tells you: at this ISO and this exposure value—which aperture, which shutter speed? Cinematographers often work with an index system that is memorized or kept as a quick table on a sticky note attached to the photometer. The higher the index (higher ISO), the smaller the aperture you can use—relevant for depth of field control.

In contrast to absolute exposure measurement, the index offers a relativistic view: it's not about absolute values, but about the correlation between light, film/sensor sensitivity, and optical control. This makes it particularly valuable in variable production conditions. Are you working with available light and low ISO? The index shows you the limits of your aperture choice. Are you shooting at night and need ISO 3200? The index shifts your possible apertures into different ranges—and thus also your aesthetic options for depth of field.

Related to concepts like exposure value (EV) and light metering, the index is often used in classic handheld workflows. In digital practice, where live view and histogram take over control, it loses importance—but conceptually remains the bridge between the optical system and sensor sensitivity. Anyone shooting on film or wanting to think analogically should understand the index: it is the quick answer to the question, "Which aperture to be sharp?"