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Higher Contrast Alexa Lut


justin T henning

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Hey Everyone,

 

I have been really digging in and attempting to perfect an on set viewing LUT. I know there are many theories behind this, and that most just view the standard arri rec.709 LUT on set. I have found this LUT to be somewhat limiting and am looking for something different.

 

I have read that many people consider the standard Alexa LUT too flat, which can lead to improper lighting when judging ratios and gradient via an on set monitor. Thus many people prefer a higher contrast lut for their base LUT.

 

My question is: does this all refer to the rec.709 LUT with the Alexa ? If you do use a higher contrast LUT on set how did you go about making this LUT? How much contrast did you add? Did you crush the blacks and maintain the highlights or apply a general s curve to increase contrast? Obviously working with a color scientist would be ideal in creating this, but I am hoping to expand my knowledge personally this this area and any guidance would be helpful.

 

Thanks.

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You are correct, the viewing REC709 LUT in the Alexa has a long shadow and long highlight curve. There is an app on the arri website called "look creator" with which you can modify the viewing LUT. But, you are limited to lift, gamma, and gain controls. That means that if you increase the contrast in the midtones, you will need to clip the highlights and the shadows in the on set preview.

 

I used to use such a LUT modification in the Alexa (looked kind of odd on set, but the mid-tones were very similar to film stock/print.

 

But, now, I'm using the built in viewing LUT un-corrected. And I'm lighting with really high contrast ratios as well. Very, un-film like.

 

I believe the newest Alexa cameras have the ability to load your own conversion LUT that you create elsewhere (without the look creator app) and if you do this you can build a custom LUT that will look as you desire. Also, on the arri website, is a page for creating post production LUTs for converting LogC to ...whatever. You can choose the amount of rolloff. 0 is a straight line, harsh curve, and 3 is the maximum soft curve (low contrast). I think "2" is closest to the built in viewing LUT, but not an exact match. I think you might be happy with a LUT with "1" for hightlights and "1" for shadows. Try it. It's fee anyways :) In the online app it will say "K" for Knee or shadows and "S" for Shoulder or highlights.

 

Be away that loading LUT modifications in an old Alexa is not very straight forward, and it may take you a while to figure out how to load it I've not yet tried loading a custom LUT in the newer Alexa camera myself.

Edited by Bruce Greene
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When I create my own look files, I shoot an 11-step DSC greyscale in Log-C, and shoot the chart again with ARRI Rec.709 baked-in to the recording, and then I go to a post house and correct the Log-C to look correct for Rec.709. When I compare that correction to ARRI's own Rec.709, the colorist and I usually find that ARRI's version is a little harsh in the shadows and has a mild yellow cast in the highlights for some reason. I certainly wouldn't describe ARRI Rec.709 as having a flat look unless you deliberately want a higher-contrast look, in which case you should create a look file with that in mind.

 

ARRI did create a flatter Rec.709 version (I think labelled LC) option in the camera for people who don't want to record Log-C but want to record Rec.709 with some flexibility to color-correct it later.

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  • 4 months later...

When creating a LUT it is worth carefully considering what look you are trying to go for and why. If you compare Arri's standard Rec709 LUT that comes with the camera and is included by default in Premiere and Da Vinci Resolve etc it is the same as the K1S1 LUT you can download from Arri's Website. This LUT does have a Log like curve in the shadows if you were to compare it to eg film. Also if you look at a DoP like Roger Deakins who uses one LUT that is slightly modified project to project you will notice his LUT also has a contrastier look espeically in the blacks in comparison to Arri's Rec709 LUT. I would highly recommend watching Steve Yedlin's display prep demo to get a sense of what it is like matching Arri Alexa to a film look. There is also an article on his site about colour science where you can see Arri's Rec 709 LUT vs Steve's film emulation. You will notice his emulation is more contrasty with a steeper roll off into the blacks, and of course the colours are (to my taste) far nicer and more film like. I personally do not like Arri's standard Rec709 for either its contrast or colour. It is very flat in the blacks which can lead to less difference in your lighting between lighter and darker areas which leads to less gradation / tones in your blacks, and especially so when grading. Rather than having a nice long gentle toe into the blacks, with the Arri LUT you can end up losing these gradual tonal shades of shadow. The Arri Rec709 can be good as a look for some things of course, it just depends on what you're going for. You can grade an image, create and export a 32x32x32 LUT in Da Vinci resolve and then create an .aml Arri Look file from this in ArriColorTool software (for 3D LUt capable Alexa's like the Alexa Mini) or load it as a 3D lut into a 3D lut capable monitor.

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My oversimplified view is that film is contrasty in the shadows and flat in the highlights and digital is flat in the shadows and contrasty in the highlights. So when creating LUT's for a film look, I tend to follow that philosophy, keep as much overexposure information as possible but let the deepest shadows fall-off to a decent black.

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My oversimplified view is that film is contrasty in the shadows and flat in the highlights and digital is flat in the shadows and contrasty in the highlights. So when creating LUT's for a film look, I tend to follow that philosophy, keep as much overexposure information as possible but let the deepest shadows fall-off to a decent black.

 

Have you tried or heard of this LUT:

 

https://www.thebrim.pictures/sucomo.html

 

To me it looks very interesting. The subtractive model looks lower contrast in general (especially in the highlights), but the way saturation behaves sort of mitigates it. The Alexa already rolls off the highlights with very low saturation but this seems to take the film look approach to an extreme, with very rich colors. It's almost like the substractive model has a built of an "HDR" feel to it.

 

A colorist on another forum made a credible "rebuilt" version if it, but the results were noisy. They probably are with the real thing, too.

 

The one thing I'd add is that the Alexa went through a lot of revisions (SUP 2.0, etc.) and to me the image has changed–perhaps gotten less flat–over the course of them. Lumetri has a bunch of Arri LUTs, a different one for the Amira from the Alexa, etc. So it's possible the image could end up flatter than anticipated based on firmware not matching the LUT.

 

But I also think if you're aiming for a really vivid "film look" that's heavily lit and are lighting with conservative ratios, using a higher contrast LUT on set could make sense? I used to light a lot better–or just differently–when I was shooting with a t2i, but I'm not really qualified to be in this discussion since my background is more post than shooting.

Edited by M Joel W
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