What Is A LUT? Why One LUT Does Not Fit All Shots

What is a LUT (and how do you use a LUT for color correction)?

March 13, 2013

Look-up Tables are mathematically precise. Understanding LUTs requires you to accept that a single LUT can NEVER be used for all your shots.


Series

Updated: August 2, 2016 โ€“ edited text for clarity.
Updated: January 17, 2021 โ€“ updated ARRI LUT generator text and link


What is a LUT(and how do you use a LUT)?

A Look-Up Table (LUT) is mathematically precise way of taking specific RGB image values form a source image โ€“ and modifying them to new RGB values by changing the hue, saturation and brightness values of that source image. A LUT can be scientifically precise (such as moving from the sRGB color space to the DCI P3 Color Space). A LUT can also be used creatively to impose a specific โ€˜Lookโ€™ on a source image, such as the Bleach Bypass look.

When reading about LUTs youโ€™d think that there must be one single perfect LUT for whatever operation youโ€™re performingโ€ฆ for instance, taking the LOG-recorded images from a Digital Cinema Camera and turning them into properly viewable High-Definition images.

Youโ€™d also think any LUT that comes from a camera manufacturer or that ships with your color grading software is a precise (and perhaps fool-proof) method of getting your โ€˜flatโ€™ LOG images into a nice rich HD image.

Not so.

The problem with understanding LUTs

What many post-pros donโ€™t understand is that there are several types of LUTs:

  • Technical LUTs โ€“ These types of LUTs are designed to transform an image from one color space / gamut to another. The end goal is to have the same image look perceptually identical on two different viewing devices. For instance: A plasma display and the output of your inkjet printer (technically, this is the goal of ACES โ€“ an in-development color space that covers the human visual spectrum โ€“ but conceptually the end-goal is the same).
  • Creative LUTs โ€“ These types of LUTs can be generated in software, allowing โ€“ for instance โ€“ completely different grading apps to share looks between them.

The problem occurs when we assume a Creative LUT is actually a Technical LUT. And thereโ€™s a broad type of common color correction LUT where this incorrect assumption happens regularly:

  • Camera LUTs โ€“ A good example of this kind of LUT is the ARRI Alexa Rec 709 LUT, designed to be applied to footage recorded to ARRIโ€™s flat Log-C recording setting. This type of LUT is really a merging of Technical and Creative LUTs. A Camera LUT seems like itโ€™s operating with Mathematical precision. And youโ€™d think Camera LUTs should work precisely the same way every time you use them. But they donโ€™t. Why? Because ofโ€ฆExposure. As the amount of light hitting the sensor changes โ€“ and the relationship between the darkest shadows and brightest highlights change โ€“ so does the effect of a LUT on that image.The end result: A Camera LUT is a creative tool designed with a specific purpose in mind. But what itโ€™s NOT designed to do is take every image recorded by that camera and make it perfect every time. Itโ€™s just not possible. Professionals new to LUTs often find this is a hard concept to internalize.

Camera LUTs are educated guesses by the manufacturer

One of the most popular Camera LUTs youโ€™ll come across is that โ€˜ARRI Alexa Log-C to Rec 709โ€™ LUT. It takes a flat Log-encoded image and expands it out to  make it look more normal to the human eye. But the one thing a LUT like this isnโ€™t, is one-size-fits-all.

In fact, not only isnโ€™t this type of LUT not one-size-fits-all, ARRI has a LUT generation web page that used to allow you to create over two dozen different types of LUTs for you to experiment with (they discontinued the old LUT generator a few years ago and settled on a single, generic LUT for their camera). If one size did indeed fit all, why did ARRI offer this many options?

So โ€“ whatโ€™s the deal? Whatโ€™s going on here?

Lets dig into how a LUT works

Over a series of Insights, weโ€™re going to dig into how a LUT works โ€“ not from a mathematical view โ€“ but from a practical, lets-apply-some-LUTs-and-see-what-happens view. Because, as a colorist, I consider most LUTs to be creative tools and itโ€™s important to understand them from a creative perspective.

In this video weโ€™ll be using a test image to explore how different exposures have a radically different effect on the same image when using the same LUTโ€ฆ Enjoy!

And be sure to leave to comments or ask questions.

โ€“ patrick

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