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Log C

ARRI's recording color space

ARRI cameras record and output images in Log C wide gamut color space. Log C images can transport all the color information and high dynamic range captured by ARRI’s camera sensors. As our cameras evolve, so does Log C - with the introduction of ALEXA 35 we launched its newest update: LogC4.

Logarithmic encoding - "Cineon" style

Images encoded with Log C (C is for Cineon; the original Cineon log encoding is based on the density of color film negative) can be identified by their flat and desaturated nature. Whites and blacks are not extended to their maximum values. A LogC4 image looks different from a LogC3 image because of their different origins: the original ALEXA sensor and new ALEXA 35 sensor have a different dynamic range and therefore different demands towards a logarithmic encoding of their images.

Log C uses what is known as scene-based encoding. The signal level increases by a fixed amount with each increase of exposure, measured in stops. This encoding, which uses an ARRI-specific wide gamut color space, is similar to files from a film scan and ideally suited to carrying image information. To correctly display Log C material on an HD or UHD monitor (Rec 709/Rec 2020) or in a digital projection (P3), it needs to be tone-mapped and transformed into the target color space. This image conversion can be performed using a 3D Look-Up Table (3D LUT).

For LogC3 LUTs (all ALEXA and AMIRA cameras – not for ALEXA 35) please visit our online LUT Generator.

For LogC4 (ALEXA 35 only), we have prepared a *.zip-archive with pre-generated LUTs in *.cube format.

Working with both ALEXA and ALEXA 35 sensors: mixing LogC3 and LogC4 on one project

Shooting with a combination of ALEXA 35 and other ALEXA or AMIRA cameras raises the question of how to handle both Log C versions on one project. We have anticipated this situation and offer a path for footage captured with the previous ALEXA sensor (LogC3) to be used within REVEAL Color Science alongside ALEXA 35 footage (LogC4) through our ARRIRAW SDK. Software using our SDK can develop LogC3 footage to LogC4. This can only be achieved with ARRIRAW footage and is not possible with Apple ProRes footage.

Target color space: video encoding

Rec 709/2020 is display-based encoding using RGB primaries specified in ITU Recommendation BT.709/2020 and can be directly output to standard HDTV/UHDTV displays without any conversion.

The material can be processed by most HD/UHD video production gear in real time, which enables short production times. Rec 709/2020 provides reduced choices in color grading and, due to a more contrasty characteristic, cannot hold as much highlight information as the much flatter Log C curve. Therefore, Rec 709/2020 is not available as a recording color space in ALEXA Mini LF and ALEXA 35 cameras.