ARRI's Log C white paper turns a curve into a formula

The image

The document is ARRI's white paper ALEXA Log C Curve, Usage in VFX, authored by Harald Brendel and revised by successive ARRI and industry contributors between 2011 and March 2017, describing the logarithmic encoding curve used in ARRI's cameras. A separate, newer ARRI LogC4 Specification, dated 23 January 2025, documents the successor curve built for the ALEXA 35's sensor.

What the documents show

The Log C white paper states that the curve maps the sensor signal corresponding to 18 percent gray to a fixed Log C value of 0.391, and gives the exact encode and decode formulas, each built from parameters that vary by exposure index between 160 and 1600. It documents three historical versions, noting that version 3, in use since the SUP 3.0 firmware release of 2011, remains current across ALEXA, AMIRA and ALEXA 65 cameras, and that ARRI provides look-up tables through its own LUT generator as an alternative to implementing the formula directly. The LogC4 specification documents a distinct successor curve, describing itself as a direct successor to the prior LogC v3 encoding, developed for the ALEV4 sensor, and lists a conversion path from LogC4 to the ACES 2065-1 color space. Neither document claims Log C or LogC4 encodes any final color appearance; both describe transfer functions for moving between a linear sensor signal and a logarithmic storage format.

Production context

The following is an editorial reading. A visual-effects or color pipeline that receives Log C-encoded footage needs exactly the information these documents supply, where 18 percent gray sits, and how to invert the curve, before any grading or compositing decision can proceed, because working directly on unconverted log values would misrepresent contrast and exposure relationships.

Reference versus imitation

The documents describe ARRI's own encoding, and their language is careful to note that other manufacturers use log formats addressing the same documented purpose without describing them here as equivalent in detail. Treating any camera's log curve as interchangeable with ARRI's Log C, or describing a graded look as caused by Log C itself rather than by the grading applied after decoding, goes beyond what either white paper states.

• Does a reference to log-encoded footage specify which manufacturer's curve and version is meant?
• Is a described look attributed to grading decisions, or incorrectly to the log encoding that preceded them?
• Has the correct version of the white paper been checked, given that ARRI has revised Log C's documentation across a decade?

Read together, the two ARRI documents establish a technical lineage, one transfer function succeeding another, each mathematically defined and downstream-convertible, and nothing about how a finished, graded image is supposed to appear.