THE BLOG

Exposure Index EXPLAINED in Cinematography!

Jun 23, 2025

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Exposure Index vs ISO: Why EI Is Becoming the New Standard

Understanding ISO in the Digital Age

ISO is often described as a camera’s “sensitivity to light.” That analogy worked well in the film era, when ISO (or ASA) literally referred to the light sensitivity of the film stock. In digital cameras, though, ISO doesn’t change sensor sensitivity—that value is fixed at the factory. Instead, ISO controls the amplification applied to the electrical signal that the sensor produces when exposed to light.

Raising ISO brightens the image but also boosts noise. The cleanest image usually comes from shooting at the camera’s native ISO—the level at which little or no gain is applied. On the Sony FX6 and FX3, dual-native ISO values of 800 and 12 800 provide maximum dynamic range with minimal noise.

What Is Exposure Index (EI)?

Exposure Index (EI) is often taught as a purely monitoring tool: it doesn’t alter the recorded image, it only changes the way the image looks on the monitor. Think of ISO and EI as two valves in a plumbing system:

ISO controls the “main supply”—the signal that is actually recorded.
EI controls a “sprinkler line”—how that same signal is displayed on your monitor or viewfinder.

Because the recording doesn’t change, EI lets filmmakers explore different looks or guide exposure decisions without baking those decisions into the footage.

Using EI in Practice

Previewing dark scenes: Raising EI brightens the monitor image, making it easier to see action in low-light environments while leaving the recorded exposure untouched.

Guiding exposure choices: Lowering EI (e.g., from 800 to 400) darkens the monitor preview by one stop. To compensate, you may open the aperture, slow the shutter, or add light—effectively overexposing the recorded image and pulling more detail out of the shadows. The trade-off? Reduced highlight headroom.

Using tools like waveform or false color, you can place middle gray—41 % IRE in Sony S-Log3—exactly where you want it. Lowering EI to 200 can recover deep shadows but leaves only about four stops of headroom above middle gray before clipping highlights.

EI and Dynamic Range: Understanding the Trade-Off

Dynamic range is the span of light values a camera can capture between pure black and pure white, measured in stops. High-end cinema cameras record roughly 15–17 stops.

On Sony S-Log3 at native ISO 800, the sensor captures 6 stops above and 9 stops below middle gray. Dropping EI to 400 shifts one stop from the highlights to the shadows—not because the sensor changes, but because you were encouraged to add a stop of light. Shadow detail once lost below pure black is now visible, while highlight latitude is reduced.

At EI 200, you gain two stops in the shadows but retain only about four stops in the highlights before clipping. Understanding this compromise helps you decide whether shadow detail or highlight protection is more important for a given scene.

The DaVinci Resolve Shift

Recent versions of DaVinci Resolve treat EI metadata as the primary exposure reference—even if you didn’t shoot RAW. Resolve reads your EI value and interprets footage accordingly, bypassing ISO. If you shot at ISO 800 with EI 400, Resolve will show an overexposed image unless you override it.

This mirrors RED’s long-standing approach, where ISO is just metadata that can be changed in post. With Sony and Resolve adopting this model, EI is quietly becoming the dominant exposure reference.

Why EI Has Replaced ISO in My Workflow

On set, I no longer obsess over ISO. I expose with EI, confident that I can tweak it in post to match what I saw on my monitor. If I rethink my exposure later, I simply type a new EI value in Resolve and the image updates—no re-shoot required.

ISO and EI are not interchangeable, but EI now leads the charge in modern log workflows. It bridges on-set intent with post-production flexibility, making it one of the most powerful tools in digital cinematography.

Final Thoughts

Exposure Index isn’t just a monitoring aid—it’s a creative and technical asset. By mastering EI, you can:

Protect shadow detail or highlight latitude on demand
Improve signal quality in low-light scenes
Future-proof footage for evolving post-production pipelines

With metadata-driven workflows becoming the norm, EI is poised to redefine exposure control. Have questions or thoughts? Drop them in the comments—and as always, happy filming.