How to apply a LUT to log footage the right way

To apply a LUT to log footage the right way, start with a technical conversion LUT that matches your exact camera and log profile, so the flat log image is mapped into your display space (usually Rec.709). Grade after that conversion, or better, make exposure and balance corrections before it, then add any creative look last and at reduced strength. Most problems people blame on LUTs, such as crushed shadows, orange skin or clipped skies, come from using the wrong LUT for the source, or stacking a creative look directly on log footage it was never built for.

Why log footage needs a LUT at all

Log profiles (S-Log3, V-Log, C-Log, LogC, Apple Log and others) record the scene with a curve that squeezes a wide range of brightness into the file. That is why log looks grey and washed out on a normal screen: it is not meant to be viewed directly. It is a container for information, not a finished picture.

Something has to translate that curve into an image your display understands. That translation is the job of a technical LUT (or a colour-space transform that does the same thing more precisely). Until the footage is converted, any judgement you make about contrast or saturation is a judgement about the wrong image.

Two kinds of LUT, two different jobs

The word LUT covers two very different tools, and confusing them causes most of the trouble.

Technical input LUTs

A technical LUT, often called an input LUT, camera LUT or conversion LUT, does one thing: it converts a specific log curve and colour gamut into a specific output space. A Sony S-Log3/S-Gamut3.Cine to Rec.709 LUT is built for exactly that source. Camera manufacturers publish these for their own log profiles; get them from the manufacturer's official site rather than a random pack.

A technical LUT is not a look. It should produce a neutral, natural-contrast image. If it does not, either the LUT does not match the source or the exposure is off.

Creative LUTs

A creative LUT, or look LUT, is a stylistic choice: a teal-and-orange push, a faded film stock emulation, a cold thriller palette. It was built by someone, on some footage, expecting a particular input. Many creative LUTs expect an already-converted Rec.709 image. Some are built to go straight onto a particular log profile. The pack or the author should say which; if it does not, test before trusting it.

The practical rule: one LUT converts, the other decorates. Never let a creative LUT do the conversion job unless it was explicitly built for your camera's log profile.

Where each LUT belongs in the grade

A reliable order, in any grading tool:

1. Correct exposure and white balance on the log image, if your tool lets you work before the conversion. Small adjustments here behave more naturally because log keeps highlight information that a converted image may already have thrown away.
2. Apply the technical input LUT (or colour-space transform) for your exact camera and profile.
3. Grade the converted image: contrast, saturation, secondary corrections, skin.
4. Add the creative look, ideally on its own node or layer so you can lower its strength or bypass it.
5. Check the result on scopes and in motion.

The key idea is that the technical conversion sits near the start, and the creative look sits near the end. Corrections you make between them are working on a sensible image. If you put the creative LUT first and the conversion afterwards, or skip the conversion entirely, you are asking a look to fix a problem it was never designed for.

How to avoid clipped highlights when converting log

The most common complaint after applying a LUT to log footage is blown highlights: a sky that turns to a flat white patch, or a window that loses all detail. A few causes, and what to do about each.

Overexposed log

Many shooters expose log brighter than the meter suggests to keep noise down in the shadows. That is a valid technique, but a standard conversion LUT expects the exposure the manufacturer designed it for. Feed it a brighter image and the top of the range runs out of room. Pull exposure down before the LUT, on the log signal, and the highlights usually come back.

LUTs are hard limits

A typical LUT maps input values to output values inside a fixed range. Whatever lands outside that range is usually clipped, and you cannot recover it later in the chain by pulling highlights down. This is the main reason to make exposure corrections before the LUT rather than after it.

The wrong LUT for the source

A LUT for a different log curve, or the right curve with the wrong gamut, can shift both exposure and colour. If the converted image looks too contrasty, too saturated or oddly tinted, check that the camera, profile and gamut all match what was actually set on set. Camera metadata or the shooting notes are the source of truth here.

Creative looks with aggressive contrast

Some creative LUTs add a steep curve. On top of an already-converted image that is fine; on top of an image that was converted too bright, it clips. Lower the look's strength, or reduce contrast before it.

Check on a waveform, not by eye

A waveform shows clipping as a flat line at the top. Look at it before and after the LUT. If the highlights are compressed but still separate before the conversion and flat afterwards, the conversion is clipping them and an exposure adjustment before it is the fix.

Colour-space transforms versus LUTs

Many modern grading tools offer colour-space transforms as an alternative to technical LUTs. Instead of a fixed table, the software converts from a declared source space to a declared output space mathematically. The advantage is precision and flexibility: you can change the output target (Rec.709, Display P3, HDR) without hunting for another LUT.

The limitation is the same as with a LUT: a transform is only as correct as the source colour space you tell it. If the clip is tagged as the wrong camera or profile, the result is wrong in exactly the same way.

Applying an input LUT in cinemaOS

In cinemaOS the technical step has its own control. Open Finisher, then Grade and Color, and set the Input LUT for the clip when the footage needs a camera transform. If the clip is already a finished display image, leave it unassigned. The short tutorial A grade in one minute shows the order: check the source, set the Input LUT only when it matches, then shape the image with COLOR and LIGHT.

A few things worth knowing:

• Download the conversion LUT for your camera from the manufacturer. As in any tool, the conversion is only correct if the LUT matches the camera, profile and gamut actually used.
• Keep the Input LUT for the conversion only, and treat any creative look as a separate grading step after it, so changing the look does not mean redoing the conversion.
• The grade includes scopes, so you can run the clipping check above while you work.
• The Lab in the Grade room shows a LUT as a 3D cube, can repair artifacts in a look, and exports a clean .cube file. It is useful for inspecting a creative LUT from an unknown source before you trust it.

A quick checklist

• Identify the camera, log profile and gamut from the metadata or shooting notes.
• Use the manufacturer's conversion LUT, or a colour-space transform, for exactly that combination.
• Fix exposure and balance before the conversion where possible.
• Grade on the converted image.
• Add creative looks last, on their own layer, at reduced strength.
• Check highlights on a waveform before and after every LUT.
• Watch the result in motion, not just on a paused frame.

Once the conversion is right across a scene, matching shots becomes much easier; see shot matching to a reference still. If you are still choosing a grading tool, the best colour grading software for Mac compares the options fairly.

FAQ

Can I apply a creative LUT directly to log footage?

Only if the LUT was built for your exact log profile. Most creative LUTs expect a Rec.709 image, so on log they produce wrong contrast and colour. Convert with a technical LUT first, then add the look.

Should I grade before or after the LUT?

Make exposure and white balance corrections before the technical LUT, because the log signal still holds highlight information. Do contrast, saturation and secondary work after it, and add any creative look at the end.

Why does my footage look too contrasty after the LUT?

Usually the LUT does not match the source profile or gamut, or the footage was exposed brighter than the LUT expects. Check the camera settings, then lower exposure before the conversion and watch the waveform.