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

The problem with understanding LUTs

What many post-pros don’t understand is that there are several types of LUTs:

• Technical LUTs – 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 – an in-development color space that covers the human visual spectrum – but conceptually the end-goal is the same).
• Creative LUTs – These types of LUTs can be generated in software, allowing – for instance – completely different grading apps to share looks between them.

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

• Camera LUTs – A good example of this kind of LUT is the ARRI Alexa Rec 709 LUT, designed to be applied to footage recorded to ARRI’s flat Log-C recording setting. This type of LUT is really a merging of Technical and Creative LUTs. A Camera LUT seems like it’s operating with Mathematical precision. And you’d think Camera LUTs should work precisely the same way every time you use them. But they don’t. Why? Because of…Exposure. As the amount of light hitting the sensor changes – and the relationship between the darkest shadows and brightest highlights change – 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 it’s NOT designed to do is take every image recorded by that camera and make it perfect every time. It’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 you’ll come across is that ‘ARRI Alexa Log-C to Rec 709’ 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 isn’t, is one-size-fits-all.

In fact, not only isn’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 – what’s the deal? What’s going on here?

Lets dig into how a LUT works

Over a series of Insights, we’re going to dig into how a LUT works – not from a mathematical view – 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 it’s important to understand them from a creative perspective.

In this video we’ll be using a test image to explore how different exposures have a radically different effect on the same image when using the same LUT… Enjoy!

And be sure to leave to comments or ask questions.

– patrick