Exposure Index: digital sensitivity setting that affects signal-to-noise ratio and embeds as metadata in RAW capture.
Technical Details
Digital cinema cameras like the ARRI ALEXA operate with a native EI of 800 but can process EI values between 160 and 3200. The RED WEAPON offers EI settings from 50 to 32,000, with its native EI at 2000. The EI value directly influences the signal-to-noise ratio: on the ALEXA, EI 1600 results in approximately 6dB more noise than at native EI 800. Modern sensors utilize Dual Native ISO technology – the C300 Mark III, for example, with native EI values of 800 and 3200, which can be switched between without loss of quality.
History & Development
The EI system evolved in the 1940s parallel to the ANSI standards for film sensitivity. Cinematographer Gordon Willis popularized the conscious "rating" of Kodak films in the 1970s, exposing stock like 5247 at EI 64 instead of ISO 100. With the advent of digital technology from 2005 onwards, manufacturers like RED and ARRI adopted the EI concept for their sensors. Since 2015, HDR development has enabled extreme EI ranges up to 51,200 (Sony VENICE) with acceptable image quality.
Practical Use in Film
Roger Deakins consistently used EI 1280 on the ALEXA for "Blade Runner 2049" to achieve the dark atmosphere without post-processing. Hoyte van Hoytema worked with Kodak Vision3 500T at EI 320 for "Dunkirk" to intentionally create overexposed, washed-out daylight scenes. The EI workflow allows for live grading on set: colorists can test different EI settings in real-time while the RAW data remains unchanged. For low-light scenes, shooting 1-2 stops above native EI is common, with the resulting noise later reduced digitally.
Comparison & Alternatives
EI fundamentally differs from ISO sensitivity: ISO defines the sensor hardware characteristics, while EI represents the software interpretation of the signals. While ASA/ISO represent standardized measurements, EI remains manufacturer-specific – EI 800 on an ALEXA does not equate to EI 800 on a RED. During RAW recording, EI functions solely as a metadata tag; the actual sensitivity is determined in post-production. ACES workflows therefore increasingly utilize "Recommended EI" values instead of fixed settings to ensure maximum flexibility in color correction.