If you shoot log, the picture on the back of your camera is not the picture you are
making. It is flat, grey and low contrast on purpose. A monitor LUT puts a temporary
Rec.709 preview on top of it so you can see roughly what you are pointing the camera at,
without touching the recording.
That is the whole idea. It changes what you see. It does not change what is written to
the card.
Why it matters on set
You can judge the frame. Contrast tells you where the eye goes. On a
flat image everything reads the same, and compositions that feel fine on set turn out
muddled in the edit.
Clients stop worrying. Anyone watching over your shoulder who is not
a colourist will assume something is broken when they see log. A preview LUT removes an
entire category of conversation.
You catch problems while you can still fix them. A colour cast from
mixed lighting is obvious in Rec.709 and nearly invisible in log. Finding it in the edit
is finding it too late.
What it is not
A monitor LUT is not a grade, and it is not a substitute for one. It is a rough
preview, usually a plain conversion with no creative intent at all.
It also does not help you expose. Exposure is judged on the false colour display or
the waveform, both of which read the actual log signal. Judging exposure by eye through a
preview LUT is how footage ends up underexposed, because the preview makes a dark image
look acceptable.
Why monitor LUTs are a smaller file
A grading LUT is normally 33 points on each axis, which is 35,937 colour values. Many
on-camera monitors and external recorders cannot load that. They want 17 points, which is
4,913 values.
The smaller version is less precise. On a five inch monitor in daylight you will never
see the difference, and that is the point: it is built for the job it does, not for
final output.
This is why our free pack ships both. The full size conversions are for the edit. The
17×17 monitor versions are for the camera and the monitor.
Where to load it
In the camera. Most cinema cameras and many mirrorless bodies take a
user LUT on a memory card and apply it to the display, the HDMI output, or both. Check
that it applies to the display only, not to the recording, unless you specifically want
it baked in.
In the monitor or recorder. Atomos, SmallHD and similar devices load
.cube files directly. This is usually the better place, because the LUT follows the
monitor rather than the camera and you can turn it off instantly.
Nowhere. If you shoot in a standard profile you do not need one, and
if you are working alone on a tripod with a waveform you may not want one either.
The one rule
Make sure the preview is not being recorded. Every camera phrases this differently and
some of them phrase it badly. Baking a Rec.709 conversion into a log recording throws away
exactly the range you shot log to keep, and it cannot be undone.
If you are not sure whether log is the right choice in the first place, we wrote about
when it helps and when it does not.