Camera Log

In production, "log" means a scene-referred transfer curve: the camera maps relative scene light into digital codes with a roughly logarithmic shape so that equal stop steps take similar code space. Middle gray is parked deliberately (often around 30-40% of code range, depending on the flavor). The picture is low contrast and low saturation on a Rec.709 monitor on purpose: not because the sensor failed, but because you are staring at the encoded signal, not the display-rendered image. Log is the cousin of the OETF story on the Transfer Function Explorer: camera-side encoding. Display EOTFs (gamma, PQ, HLG) answer a different question. Confusing the two is how log gets "corrected" with a contrast knob until the highlights are gone.

The industrial ancestor is Kodak Cineon (early 1990s): scanning film negative into a log density domain so digital intermediate could hold the negative's latitude without wasting codes on empty bright values. Printing density was already near-log; the scan encoded that world in 10-bit files with a defined black/white code range (the famous 95-685 neighborhood that still haunts conversion LUTs; see Anatomy of a LUT). Digital cinema cameras inherited the same problem with a different sensor: linear photon counts span many stops, but 10- or 12-bit recording is finite. Pure linear encoding spends almost all codes on the brightest stop and starves the shadows. A log (or log-like hybrid) curve reallocates codes per stop, preserving highlight headroom and shadow texture for the grade. Vendor logs (LogC, S-Log3, V-Log, REDLogFilm / Log3G10, and the rest) are productized answers to that quantization problem, plus marketing and pipeline lock-in, not seven different religions of light.

Record: the camera writes log (or log-in-a-container) to maximize latitude and to keep a consistent encoding for dailies and archive. Monitor: on-set monitors almost never show "honest log." A technical conversion LUT (log to Rec.709, or log to HLG/PQ for HDR monitoring) or an exposure tool (waveform in the right mode, false color keyed to the log's middle gray) makes the image readable; that is the same "most-used LUT on earth" class in LUT Anatomy. Grade: the colorist works scene-referred (decode log to linear or ACES, then render out) or in a log-aware toolset. Creative look LUTs are authored for a declared input; feed LogC into a Rec.709 look and teal lands in the wrong place. Deliver: broadcast and streaming still want display-referred Rec.709 / PQ / HLG. Log is a capture and intermediate language, not a delivery standard (except when a facility deliberately ships log mezzanines under a locked pipeline).

x is scene-linear relative exposure (proportional to luminance above sensor black, scaled so the curve's reference mid-gray is consistent in software). y is normalized code (0-1), sometimes shown ×1023 as 10-bit. Piecewise curves use a linear toe so the derivative stays finite near black; pure log(0) is undefined and would waste precision. When you implement transforms for money, prefer manufacturer CTL/DCTL/Cube or ACES IDTs; the explorer is for learning shapes, mid-gray placement, and bit allocation.

Not raw: log is already demosaiced (or at least image-formed) RGB or YCbCr with a tone curve; raw keeps sensor samples and defers that work. Not "more dynamic range than the sensor": log cannot invent stops the photosites did not capture; it only spends bits better across the stops you have. Not interchangeable without math: "it looks flat" is not a conversion, and cross-walking LogC3 to S-Log3 without the published curves (or a verified matrix plus curve) is a gamble. Not a free pass on exposure: ETTR still matters, log's highlight shoulder is finite, and clipping in log is still clipping, just later on the waveform if you know where to look.

Cinema often records camera log or raw+log proxies with a DIT and a show LUT stack. ENG and hybrid documentary kits use the same curves (S-Log3, C-Log3, V-Log, F-Log) but may go live or to a same-day cut. The failure mode is identical: monitoring Rec.709 while recording log without a technical LUT, or shipping a look LUT built for the wrong log. If the camera can record log, the truck must understand log.

Log pipelines fail when metadata and monitoring lie: wrong input LUT on a client monitor, an SDR legalizer chewing headroom, or a probe-driven display calibrated for Rec.709 while the cart is feeding log. Calibration still measures display light (EOTF, white, gamut). Log is the signal story upstream. The clean split, scene-referred capture and display-referred mastering, is the same philosophy as PQ vs camera OETF on the Transfer Function Explorer.