Log-C

Overview

Log C (logarithmic, C for Cineon) is the logarithmic image encoding developed by camera manufacturer ARRI. It is used in the ALEXA, AMIRA, and ALEXA Mini cameras and is conceptually based on the Cineon curve from the film scanner world. Instead of compressing the image internally to a display standard like Rec 709, Log C stores a flat, desaturated signal that preserves the full dynamic range and color information of the sensor. A characteristic feature is the logarithmic relationship between exposure and the encoded signal: each additional stop of exposure increases the signal by the same amount over a wide range. This prevents early clipping of highlights and preserves detail in the shadows – behavior similar to that of an exposed film negative.

Important: Log C is a recording and encoding technique of the camera, not a look. A Log C image appears flat and low in contrast straight out of the camera and must be converted via a LUT (Look-Up Table) or a color transform to Rec 709 or Rec 2020 for evaluation on set and for final post-production.

Versions: LogC3 and LogC4

There are two current generations that are not compatible with each other. They differ because the underlying sensors (ALEV3 and ALEV4, respectively) have different dynamic ranges and thus different encoding requirements.

According to ARRI, LogC3 maps approximately 14 to 15 stops, while LogC4, together with the ALEV4 sensor of the ALEXA 35, utilizes an extended dynamic range. The lower middle gray value of LogC4 causes uncorrected LogC4 footage to appear darker than LogC3 – this is intentional and creates more headroom for highlights.

Usage On Set and In Post

In practice, Log C means: the camera records flat, but a display LUT (Viewing LUT) is applied to the monitor so that the Director of Photography, Gaffer, and Director can evaluate a realistically contrasted image. This LUT does not alter the recording – the Log C negative remains untouched and is only developed during color grading.

• Exposure: With LogC3, 18% middle gray is at around 39% signal, which serves as a reference when applied to a waveform or false color display. With LogC4, the reference value is significantly lower at approx. 28%.
• Monitoring: The correct Log C variant (C3 vs. C4) and the appropriate LUT must match; LogC4 footage will not produce a usable image without the correct transform.
• Grading/VFX: Log C is converted via a color transform into the working color space (e.g., ACES) or via a Rec 709/Rec 2020 LUT into the output color space; ARRI provides pre-made LUTs as well as an online LUT generator.