Log recording and LUTs — what happens to the picture
Log isn’t a ‘look’. It is a way of squeezing more dynamic range into the signal than ordinary video can hold — at the cost of having to translate the picture before you use it.
The problem: the sensor sees more than video can carry
Modern sensors capture a range from deep shadows to bright highlights that goes far beyond what an ordinary (SDR) television can display. The standard HDTV transfer curve from the ITU-R BT.709 recommendation is designed for a picture that is viewed directly: the midtones get plenty of code values and bright highlights hit the ceiling fairly early. Anything above the ceiling is clipped and lost for good.
A log curve distributes code values more evenly across the stops of exposure. Each stop of light gets a similar ‘slice’ of the signal. The result: both highlights and shadows fit into the recording, but the picture looks flat and desaturated, because the contrast that a display curve would create is missing.
DIAGRAM · TRANSFER CURVES
The graph shows the essence: the Rec.709 curve reaches 100% of the signal about 2.5 stops above middle grey. At that point the log curve is still rising and leaves room for further stops of highlights.
What log needs
Bit depth. When the same range is spread into less contrast and then ‘stretched’ again in the grade, jumps appear between neighbouring values. At 8 bits (256 levels per channel) this shows up as banding in skies and smooth gradients. That is why log belongs in 10-bit or higher recording. Many cameras also offer log in 8 bits — it can be done, but it is risky.
Correct exposure. Log is more forgiving in the highlights, but its shadows have few code values. Underexposed log gets noisy faster than you would expect when you lift it. Manufacturers give a target level for middle grey for each curve — expose to it and measure with a waveform.
A transform before viewing. Without translation into a display space, log is useless for judging colour and contrast.
What a LUT is
A LUT (look-up table) is a table that maps each input colour to an output colour. A 1D LUT changes each channel separately (a luminance curve, for example). A 3D LUT works with the combination of R, G and B — it can change hues and saturation too, for example shifting greens without changing skin tones. A 3D LUT with a 33 × 33 × 33 grid contains 35,937 points; the colours in between are calculated by interpolation.
It is important to distinguish between two kinds:
| Technical (conversion) LUT | Creative LUT (‘look’) | |
|---|---|---|
| Purpose | Conversion from one colour space and curve to another (e.g. log → Rec.709) | Stylisation: contrast, colour tone, a ‘filmic’ look |
| Source | The camera manufacturer, a standardised workflow | Colourists, vendors, your own work |
| Input | Precisely defined (a specific curve and gamut) | Often designed for a specific input — outside it, it behaves unpredictably |
| Where in the chain | At the start, or as the output transform | Only after balancing and correction |
Where to use a LUT
On set (a monitoring LUT). The camera or monitor applies the LUT only to the preview; the recording stays log. The director and the cinematographer see a picture close to the final result, and exposure can be judged in context. Check in the menu whether the LUT is ‘baked in’ (burnt into the recording) or applied to the output only — a mistake can’t be undone.
In post-production. A typical order of nodes or layers:
- Input transform — from the camera’s log and gamut into the working space (a technical LUT, or better, a mathematical colour-management transform that doesn’t lose out-of-range data).
- Correction — white balance, exposure, shot matching.
- Creative look — including a creative LUT if you use one, often at reduced strength.
- Output transform — to Rec.709 for SDR, or to PQ / HLG for HDR.
LUTs clip: anything outside their input range (0–1) is usually cut off. If you push a shot into the highlights before the LUT, the LUT will clip them. Do corrections that need the full range before the LUT, or use colour management instead of a LUT.
The most common mistakes
- The wrong input LUT. A LUT for S-Log3/S-Gamut3.Cine applied to S-Log3/S-Gamut3 gives subtly shifted colours that are hard to track down. Check the names exactly.
- A creative LUT straight on log. If it was designed for Rec.709 input, the result will be blown out or far too contrasty.
- Grading on an uncalibrated display. The LUT does exactly what it should, but you see it through a monitor that may have a different gamut, curve and brightness.
- 8-bit log and heavy adjustments. Banding on skies and walls. If the camera can do 10 bits, use them.
- A monitoring LUT accidentally baked into the recording. The most expensive mistake — the recording loses its log headroom.